A myxozoan survey was performed on specimens of thicklip grey mulletChelon labrosus(Risso) captured from the Douro River estuary, northern Portugal. Eleven new species, all belonging to the genusMyxobolusBütschli, 1882 (M. abdominalisn. sp.,M. aestuariumn. sp.,M. caudalisn. sp.,M. chelonarin. sp.,M. cucurbitiformisn. sp.,M. douroensisn. sp.,M. intestinicolan. sp.,M. invictusn. sp.,M. labicolan. sp.,M. peritonaein. sp., andM. pinnulan. sp.) are described based on microscopic and molecular data, confirming the known high radiation of these myxozoans in mullets. Additionally,Myxobolus pupkoiGuptaet al., 2022 is reported for the first time fromC. labrosus, bringing forth a novel case of morphological plasticity between geographic isolates. We consider that molecular-based comparisons are imperative for the description of mugiliform-infectingMyxobolus, with distance estimation further matching two of the novelMyxobolusspp. with sphaeractinomyxon types previously reported from another Portuguese estuary. This finding supports sphaeractinomyxon as specific life cycle counterparts ofMyxobolusthat infect mullets. Phylogenetic analyses of 18S rDNA retrieved a monophyletic clade of mugiliform-infecting myxobolids comprising well-supported lineages of species parasitizing mullets from the generaChelon,Mugil,Crenimugil, andPlaniliza. The existence of more than oneChelon- andPlaniliza-infecting lineage reveals that myxobolids parasitized members of these genera multiple times during their evolution. Lastly, the elevated number of unmatched sphaeractinomyxon sequences included in theChelon-infecting lineages clearly shows thatMyxobolusdiversity hosted by this genus remains underrated.