Members of the Polymorphidae (Acanthocephala) are distributed worldwide as endoparasites of marine mammals, fish-eating birds, and waterfowl. The family contains 10 genera, with approximately 127 species. Polymorphids are characterized by having a spinose trunk with a bulbous proboscis, double-walled proboscis receptacle, long lemnisci, and 4 tubular cement glands. The taxonomic position of several genera within Polymorphidae has been controversial when considering morphological and ecological characters. The mitochondrial coding gene cytochrome-c oxidase representing species of 5 genera of polymorphids (Corynosoma, Lühe, 1904, Hexaglandula Petrochenko, 1950, Southwellina Witenberg 1932, Polymorphus Luhë, 1911, and Profilicollis Meyer, 1931) were sequenced to determine the sister-group relationships among 2 particular genera, i.e., Hexaglandula, and Profilicollis. Maximum likelihood and Bayesian analyses showed that Polymorphidae is a monophyletic assemblage, and that 3 major clades are present. Our results provide support for the idea that Hexaglandula represents an independent lineage, whereas, in the case of Profilicollis, there is no conclusive evidence that they are not members of Polymorphus. The analyses also confirm that Polymorphus is paraphyletic, suggesting that the genus represents a complex of species that should be re-examined and reclassified using morphological, ecological, and molecular data. Our observations suggest that decapods (intermediate hosts for the 2 genera under study) were independently colonized at least twice during the evolutionary history of the group.