The creature took on the host's memories, appearance, abilities, everything. It sucks everything dry. Utterly terrifying. Can you not see that if a mimic such as this were to infiltrate a population, it could replicate almost indefinitely! With evil taking the form of an ally, the galaxy could be overthrown. It is truly the worst kind of evil... it has no name, but... we call it X.

Old Bird, describing the X Parasite

The X Parasite, also known as Unidentified Parasite Creature-X[1], is an extremely hostile, shape-shifting organism native to planet SR388. They are witnessed in the prequel manga, and make their resurgence the post-credits scene of Metroid: Samus Returns. The X are later discovered and eradicated during the events of Metroid Fusion. They kill and assimilate other life forms in order to survive and spread; though simple-minded and entirely driven by instinct, they are in fact a threat to all natural life in the cosmos if left unchecked.

Their very existence is what drove the ancient Chozo race to create the titular and dreaded Metroid species with the intention of wiping out the X Parasite, therefore triggering most of the Metroid series' events.

Biology and CharacteristicsEdit

Core-X Artwork


The appearance of an X Parasite's pure form is simple: a basketball-sized, floating gelatinous organism. The structure of their bodies allows them to slip through the tiniest pore of any creature or crevice of any synthetic surface; their soft form also allows projectiles to harmlessly pass through them (with the exception of the Blue X due to its more solid body). Additionally, they have the ability to multiply rapidly via asexual reproduction, leading to infestations.

The X Parasites constantly hunt for sustenance. When a prey is located, they infect the organism's nervous system where they start reproducing at an exponential rate, resulting in the death of the host creature. It is during this process that the X absorb the DNA of their victims and copy their memories, allowing the parasites to imitate their prey's forms, behaviors and use their newfound knowledge to further their own species' existence.

Once they inhabit the body of their target, one of the following three events can happen:

  • The X take complete control over its host's body, often mutating and rendering it aggressive by altering the victim's DNA. This turns the host into an effective carrier and will spread the infection to other organisms. These changes also serve to increase its chances of survival if a Metroid attacks it. Uninfected creatures within the BSL station are normally docile but react adversely to Samus once an X infects them due to the presence of Metroid cells within her.
  • The X immediately digest their host, melting away all soft organic material in a matter of seconds. A Hornoad in the Metroid Manga and presumably the Serris within the BSL suffered this fate.
  • The X induce a coma in their host and slowly kill their victim. This happened to Samus Aran, though the exact reasons are unknown. Her Power Suit may have provided a small amount of protection, preventing the X from killing her instantly.

Whichever of these events occur, X Parasites eventually kill their host and leave the body with a copy of the latter's genetic information. At this point, they will either remain in their pure form or use the newly acquired DNA and memories to mimic their recent victim, blending in with nearby life forms. Imitating an organism's body facilitates the hunt for additional prey, as it allows the parasite to approach unsuspecting targets before striking. SR388's wildlife is sometimes known to flee at the sight of an X's true form, though the latter is still capable of giving chase with its flying, gelatinous body.

If an X Parasite that carries DNA makes contact with either another host or a fellow X carrying a different set of genes, it may result in the combination of multiple lifeforms' biologies and create unnatural monstrosities. If an infected individual or imitation is destroyed, the pure form of X are released or revealed respectively and will proceed to infect the attacker if possible. If infection is impossible, it will either flee the vicinity or transform again; in the latter case, it may replicate the same body repeatedly or the forms of other past victims. In some cases, the X will simply remain in place after losing its host or mimicry, possibly stunned or exhausted from the battle.

There are rare times when the X will ignore potential prey. For example, Dachoras and Etecoons found in the Habitation Deck, an X-infested area of the BSL station, were left uninfected. Strangely enough, Adam suggests that the X believed these creatures were too weak or inferior for their use. After being freed and retreating in Samus Aran's Gunship, further exposed to X-infested environments, the creatures remained ignored throughout the remainder of their time on the BSL.

One notable example is the SA-X, an X Parasite that mimics Samus and her entire Power Suit. Though the parasites in Samus' body were killed by the Metroid Vaccine, several components of her suit still carrying X particles were removed before the vaccine was administered. These infected parts not only replicated the missing armor pieces, but Samus' body as well, allowing the SA-X to begin attacking the BSL research station using both the suit's full arsenal and Samus' experienced skills. The X can also "download" data organically, as witnessed by Samus when she came upon a Core-X attached to a Data Room's central machine containing the Varia Suit upgrade. It processed the upload and became the Barrier Core-X, though its defeat allowed Samus to recover the upgrade by absorbing the X. The SA-X and the Barrier Core-X indicate that the species can assimilate, manipulate and reproduce most, if not all, technology in an organic matter; this specific ability seems to have driven much of the Galactic Federation's interest in the X Parasites, as the organization is known to struggle greatly when recreating the Chozo's advanced tech.

The X have great versatility and adaptability in any environment, including those that are alien to them. They effortlessly invaded and spread out in the numerous sectors found in the BSL Station; these sectors are replicas of environments found on other planets and contain climates that do not exist in their homeworld of SR388. To make matters worse, unlike the wildlife that evolved on SR388, organisms from alien planets are entirely oblivious to the dangers of the X and are less likely to flee at the mere sight of one, therefore increasing the parasites' rate of infection.

MSR Chozo Memory 06

Several Metroids attacking the X Parasites of SR388

The only predator of the X Parasites are the Metroids, which were created by the Chozo for that sole purpose. To defend themselves, the X would fight using their hosts, transform into imitations (both natural and unnatural) or develop protective shells. Overall, these defenses had little to no effect on Metroids and the X population on SR388 was reduced to near extinction. The events of Metroid Fusion revealed the X are not above sacrificing some of their own in an attempt to destroy their predators and secure safety for the species as a whole.

Taking into account their aggressive method of infection, their adaptability, rapid asexual reproduction, along with the assimilation of memories and their understanding of technology, the X Parasites are arguably a greater threat to the cosmos than the heavily feared Metroids. If an X gained access to a spacefaring vessel with the memories of an able pilot, it would travel from planet to planet, effectively becoming a galactic plague that would result in the demise of every natural form of life.

Early HistoryEdit

MSR Chozo Memory 04

The Chozo coming across the X Parasites.

Prior to the events of any Metroid game, the X Parasites evolved on SR388, where they began to greatly endanger the planet's ecosystem by throwing it out of balance, as no creature in SR388 was capable of predating on them while the X itself could successfully infect any living being. The Chozo discovered the X while exploring and building their colonies on the planet; they were the ones who first named the organism and discovered that the X could "borrow forms from their companions". This ability horrified the Chozo enough to consider the X an "evil".

MSR Chozo Memory 07

The Metroids keeping the Chozo safe from nearby X.

Recognizing the X's destructive potential, the Chozo created the Metroids in order to keep them at bay. This is first explained in the manual for Metroid Fusion and is later further explored in Volume 4 of the official Metroid Zero Mission Manga and the Chozo Memories feature of Metroid: Samus Returns. In the manga, the Chozo refrain from telling the Galactic Federation of the X Parasites' existence, fearing that someone would view the parasite's abilities as a 'splendid technology' and use it for themselves; this fear would come true during the events of Metroid Fusion, when the Federation took an interest in the SA-X, and decided to use it and the X for military purposes.

In Metroid II: Return of Samus and its remake, Metroid: Samus Returns, Samus wipes out all of the Metroids on SR388, unknowingly allowing the X Parasites to make a recovery in their population and return in Metroid Fusion. In a post-credits scene for Metroid: Samus Returns an X Parasite manages to engulf a Hornoad and proceeds to infect it.

Samus and the XEdit

As soon as the Federation ships land, the X will overwhelm them and absorb the knowledge and power of their occupants. The X are just waiting for the hunt to begin, and then they'll spread across the universe. Galactic civilization will end.

The X hunger for form, knowledge, and power. They mimic these perfectly. But they cannot copy the soul. They're single-minded, instinctively seeking to increase in number. They're a plague, and the Federation underestimates their threat.

The X must not leave here. I must destroy them all before the Federation arrives. This station has a self-destruct mechanism. I must use it to destroy the X here and on the planet. I must send them to oblivion. Them, the station, and myself, if I have to.

Samus Aran, musing on the hazards of the X Parasite

The X were first encountered on SR388 as Samus aided Biologic's research team in capturing native life forms; when Samus killed what was thought to be an ordinary Hornoad that attempted to attack the group, an X parasite came out of the carcass. The X promptly attached itself to Samus, but as she suffered no ill effects from the encounter, she thought little of it and continued escorting the researchers. However, the X soon infested Samus's central nervous system and later rendered her unconscious while she was in her gunship. Had it not been for the emergency systems in Samus's ship that automatically ejected her before the ship crashed into an asteroid belt, Samus would have died then and there. The B.S.L. scientists recovered Samus' escape pod and brought her to Galactic Federation Headquarters for treatment.

Though Samus had escaped her gunship's destruction, she was still infected by the X, which had now spread to various portions of her Power Suit, prompting the Federation medical personnel to surgically remove the infected parts. The infected pieces of the Power Suit were sent to the B.S.L. station for further study, where they would eventually become the monstrous SA-X. However, the medics could not remove the X embedded in Samus' central nervous system without killing her, and they had no way of counteracting the infection; Samus was given a very small chance of survival. Fortunately, the Galactic Federation scientists were able to devise a vaccine using a cell culture from the last infant Metroid. As the Metroids had been designed to be the perfect natural predator of the X, the parasites in Samus' body were quickly and completely eradicated by the Metroid Vaccine. As a side effect of the vaccine, Samus' genetic makeup now included Metroid DNA, granting her the ability to safely absorb the X like the Metroids could, but also leaving her exposed to the cold (A Metroid's weakness) as well as being much more vulnerable when attacked until she absorbed the Varia upgrade.

Shortly after Samus recovered from the infection, the Federation received a distress signal from the B.S.L. station. Samus was sent to investigate after the Federation provided her with a new suit, a new gunship, and a computerized CO to assist her in her investigations. Once Samus arrived, she found the station completely overrun by the X.

At numerous points, the X attempt to stop Samus in her tracks; they use methods such as hunting her down, mutating into newer, stronger forms, and intentionally destroying or blocking off Data Rooms to prevent her from recovering lost abilities. On one memorable occasion, the X went so far as to attempt to destroy the entire B.S.L. station along with Samus and themselves.

Apparent destructionEdit

At the end of this adventure, Samus flies the self-destructing Biologic Space Laboratories research station near SR388 before evacuating, destroying both the space station and the planet and presumably wiping out the X forever.



An X-Parasite scanned by a security sensor.

Yellow XEdit


Yellow X

Yellow X are a basic type of X. When absorbed by Samus, they restore 10 units of energy. Yellow X come out of most normal enemies. A Yellow X was the X that infected Samus on SR388 after it flew out of a defeated "Hornoad".

Green XEdit


Green X

Green X appear once Samus regains her Missile Launcher. Like Yellow X, Green X come out of most enemies. When Samus absorbs a Green X, two Missiles are restored.

Red XEdit


Red X

Red X are usually only released when X-infected Gadora or certain variations of normal enemies (usually of a different color and harder to defeat) are killed, although they can also rarely appear from normal enemies. They are the least common non-boss X. When Samus absorbs them, they restore 500 units of Energy, 10 units of Power Bomb Ammo and 50 units of Missile Ammo.

Blue XEdit


Blue X

The Varia Suit data is here from HQ, ready to download. Get moving to a Data Room. But there's a problem... X from the sub-zero sector ARC are in Sector 6 (NOC), and they... They've been changed by the cold. If you absorb one, you'll be frozen from within. This seems like a deliberate tactic. Once you download the Varia Suit modification, you'll be out of danger. Just be extremely careful until then. The NOC Data Room is here. Remember, avoid the blue X. Use your beam to stun them in order to get by.


Blue X (called Cold X in Japanese) are blue-colored X Parasites that have been changed by the cold in Sector 5 of the BSL station. Apparently, the X did this deliberately to both stop Samus from exploring the station and kill her.[2] If Samus comes into contact with these X without the Varia Suit upgrade, she will unwillingly absorb them and be dealt a large amount of damage. This is due to the Metroid vaccine as Metroids are susceptible to ice, thus when she absorbs one of these X Parasite variants, she will be frozen from within. They are larger than most types of X, and they are the only kind that do not take the form of another creature.

When Samus first encounters them in Sector 6, they are eager to be absorbed by Samus, and pursue her to achieve this goal. Samus must avoid them or else face death. She can fire her beam or Missile weapons to temporarily stun the parasites, allowing her to escape in the event that she is spotted by one. Once she reaches the Data Room in the sector and defeats the Barrier Core-X, she obtains the Varia Suit. When the Varia Suit is acquired, the Blue X can be safely absorbed, and they happen to restore quite a bit of energy. Immediately after obtaining the Varia Suit, several unsuspecting Blue X will fly towards Samus in the next room. After several have been absorbed, any remaining will noticeably start to avoid and fly away from Samus, and all existing Blue X within the BSL station will now flee from Samus as soon as they detect her. They can still be stunned, and recover more health for Samus than ordinary yellow X (30 instead of only 10). These only appear in Sector 5 and Sector 6.

Interestingly enough, they are much larger in size as opposed to the yellow, green and red X. Strangely, they are unaffected by the Power Bomb's vacuum ability.

Core-X Edit

An X Parasite that either carries the DNA of a massive and powerful life form, or has absorbed advanced technological equipment. It develops a protective and hazardous shell that produces regular X Parasites when attacked.

Official dataEdit

Metroid Fusion websiteEdit

"A parasitic virus native to SR388 that can perfectly replicate its host's powers. Initially contained for research, an explosion has released the parasite into the station's numerous breeding environments. A special vaccine allows Samus to replenish energy by collecting X Parasites."

Other appearances Edit



Super Smash Bros. UltimateEdit

SSBU X Parasite Spirit Board

X Parasite as a Spirit

In Super Smash Bros. Ultimate, the X Parasite appears as Primary Spirit, which boosts fighters' stats in game modes such as the World of Light. It is represented with artwork of three Yellow X clustered together. The X Parasite's type is Attack, which gives it advantage over Grab Spirits but is weak to Shield Spirits, and its class is Advanced. Additionally, upon leveling up the X spirit to Level 99, it can be enhanced into the Nightmare Primary Spirit, which grants the wielder lifesteal in combat.

The X Parasite can be obtained through the Spirit Board. When it appears, the X Parasite uses Kirby as a Puppet Fighter, most likely since they are both blob-like beings that consume other creatures and copy their abilities. The player must defeat a team of ten Kirby Puppet Fighters appearing in their yellow "Keeby" alternate costume. Each Kirby is more likely to use their Inhale move to copy the player's character. The Spirit Battle takes place on Lylat Cruise, with Sector 1 as the background music.

Non-canon warning: Non-canonical information ends here.


  • In one event, one X attempt to overheat the Station, which not only would have killed Samus but also themselves. When speculating on the reason for this action, Adam states that the parasites must have felt Samus was a major threat to their entire species on SR388 and that the sacrifice of the X on board the BSL would have spared their brethren on the planet.
    • Similarly, one SA-X destroys the Restricted Laboratory at the cost of its own life. As the laboratory contained Metroids, the X's natural predator, the SA-X's sacrifice would have spared its brethren on the BSL from the Metroid threat, although this failed as one had escaped.
  • Despite the X having gained hacking capabilities from their human hosts, they never attempted to open all the security hatches and invade all the sectors. It is also unknown why they disabled the entire station altogether.
    • It is possible they did this to prevent Samus from doing any further damage to the rest of the species on board the station by locking Samus out of the rest of the station. Curiously, the X never attempted to re-lock the security hatches; it is possible that the X never learned the location of the Security Rooms, which are not on any maps on the station, thus preventing them from locking and unlocking the different hatches.
  • The X-Parasites are similar to the Ing from Metroid Prime 2: Echoes in a few ways. Both can occupy the bodies of other beings, but the Ing cannot replicate/assume the form of a creature they have possessed and the X can neither directly possess organisms nor infect purely mechanical constructs. They are both the menace of a higher race (the X to the Chozo, and the Ing to the Luminoth). Lastly, both have been associated with and/or copied some of Samus's abilities (the sub-guardians for the Ing and the SA-X and Core-X for the X-Parasites). One important advantage the Ing have over the X is their capacity for possessing Metroids, the natural predator of the X. This would seem to hint that the X can only infect natural organic tissue (the Metroids are synthetic lifeforms created by the Chozo), while the Ing can possess any sentient or non-sentient, organic or inorganic being from the light dimension.
    • Another advantage that Ing have over X is that the latter are incapable of performing any sort of combat in their natural form, while Ing are, if anything, even more dangerous in their natural form, taking into account Dark Aether's toxic atmosphere.
    • Like the Ing, the X appear to be able to infect and control deceased bioforms, as they are able to resurrect the deceased clone of Ridley and possibly Nightmare, both of which seemingly died in Metroid: Other M; this is similar to the Ing's possession of dead Galactic Federation troopers to create Dark Troopers. It should be noted however that Ridley's clone, which was frozen at the time of X infection, was only temporarily resurrected and quickly died again shortly before the Core-X left the body, causing the body to fall apart.
  • The X-Parasites often tend to make the creatures that they mimic or infect more fearsome in appearance and often add abilities and properties that aren't found naturally in their victims. Examples of this include Neo Ridley's oversized figure and Arachnus's sonic blasts. These changes are likely made through DNA manipulation, and may involve combining the abilities of multiple creatures that they have absorbed.
  • The X resemble a species found on Zebes in Super Metroid called Boyon. In beta trailers for Fusion, the X used recolored Boyon sprites.
  • Power Bombs can pull all free-floating X Parasites within range (except for Cold X) into the center of the blast from the vacuum of the explosion.
  • The X are the first shapeshifters ever to appear in the Metroid universe. Later characters and creatures bestowed with this ability are the Mimic in the prequel manga, Dark Samus in Metroid Prime 2: Echoes, Gorea in Metroid Prime Hunters and Gandrayda in Metroid Prime 3: Corruption.
    • Ironically, the X are the last to appear chronologically.
  • The alien organisms in John Carpenter's 1982 film The Thing have an identical function to the X parasites. The organisms are able to infect/absorb and mimic their victims and also mutate when threatened (albeit in a more gruesome and graphic manner). One major difference between the X and the Thing seem to be with their instincts on preservation, as individual X parasites are much more selfless and willing to sacrifice themselves for the greater survival of their whole species against a huge threat, while individual Things are more selfishly driven with the willingness to attack and kill their own kind for the sake of their own self-preservation in maintaining their anonymity among potential prey.
    • Similarly, the X share characteristics with the Pod People from the 1956 and 1978 films, Invasion of the Body Snatchers, which survive by duplicating, mimicking, and ultimately supplanting the dominant species of any planet they come across. They also share the 1978 version's gelatinous form.
  • During the events of Metroid II: Return of Samus and Metroid: Samus Returns, SR388 seems to have a relatively low population of wildlife. The X Parasites are seemingly responsible for killing most of the bioforms inhabiting the planet, as stated by Yoshio Sakamoto and the Metroid manga. This would also explain why, in Metroid Fusion, the BSL Station's SRX (a sector that perfectly replicates the planet) contained very few of SR388's wildlife seen in the earlier game; after the extinction of Metroids, the X undoubtedly resurfaced in large numbers and steadily reduced the fauna on the planet even further (if not completely), making Galactic Federation Researchers' task of collecting varied specimens a difficult one. Whether or not Metroids also contributed to this reduction of wildlife populations before Metroid II and Samus Returns is unknown.
  • The Blue X's newfound cold nature in Fusion could have given the X Parasites back on SR388 an advantage against the advanced Metroids initially inhabiting the planet, as the latter can be wounded directly with cold-based weapons (shown in both Metroid: Samus Returns and Metroid Fusion). Although the planet seemingly lacked a naturally occurring cold climate, the Chozo laboratory (initially blocked off by the Purple liquid) had snow and ice present within a few rooms. It is unknown if the X Parasites ever reached the lab at some point, nor if the rooms would have been sufficient to create Blue X.
    • Dealing with the Larva Metroids would have required additional effort, as the Blue X would be unable to shatter them after encasing them in ice, as they lacked concussive weaponry. On the other hand, mimicking a large, powerful SR388 creature like Arachnus might have given them the firepower to shatter them.
  • As previously mentioned, the Blue X is larger and can be directly shot. These unnatural traits might be directly caused by its cold characteristics, such as having a more solid/thicker form than other X (logically having its liquid structure being partially frozen) and an expanded body (similar to how water expands when frozen), both of which consequently make the organism more susceptible to being shot at.



  1. ^ Metroid Fusion manual, pg. 11
  2. ^ Adam: The Varia Suit data is here from HQ, ready to download. Get moving to a Data Room. But there's a problem, X from the sub-zero sector ARC are in Sector 6 (NOC), and they've been changed by the cold. If you absorb one, you'll be frozen from within. This seems like a deliberate tactic. Once you download the Varia Suit modification, you'll be out of danger. Just be extremely careful until then. The NOC Data Room is here. Remember, avoid the blue X. Use your beam to stun them in order to get by. Nintendo R&D1. Metroid Fusion. Nintendo. English. 2002-11-17.
