Simple SummaryThe mature spermatozoon ofPeracreadium characis, an intestinal parasite of the sheephead bream, is described by means of transmission electron microscopy (TEM). It shares the common ultrastructural features described in the majority of digeneans (two 9+‘1’ axonemes, mitochondria, nucleus and parallel cortical microtubules). The absence of external ornamentation of the plasma membrane and spine-like bodies distinguishes the spermatozoon ofP. characisfrom the spermatozoa of the remaining studied opecoelids.AbstractThe spermatozoon ultrastructure ofPeracreadium characis(Stossich, 1886) (Digenea: Opecoelidae), an intestinal parasite of the sheephead breamDiplodus puntazzo(Walbaum, 1792) (Sparidae), is described by means of transmission electron microscopy (TEM). The mature spermatozoon possesses two axonemes of the 9+‘1’ trepaxonematan pattern, an anterior electron-dense material, two mitochondria, a nucleus and parallel cortical microtubules distributed in two bundles. The absence of external ornamentation of the plasma membrane and spine-like bodies are the noteworthy characters that distinguish the spermatozoon ofP. characisfrom those of most opecoelids. In fact, onlyHelicometra fasciatalacks external ornamentation in the spermatozoon. A comparative study with the remaining opecoelids described so far reveals similarities in the ultrastructural organization of their sperm cells. In addition, the current data on sperm ultrastructure in species of the recognized opecoelid subfamilies are compared, namely the Hamacreadiinae, Helicometrinae, Opecoelinae, Opistholebetinae and Plagioporinae.