Polymorphus arctocephali (Acanthocephala: Polymorphidae) was differentiated from P. cetaceum based on patterns in the trunk spine distribution and slight morphometric differences. The comparison of both species involved samples from South Australia and did not include P. cetaceum from South America. In this paper we re-examine the systematic position of P. arctocephali based on a more detailed morphological and geographical analysis. Results indicate that P. arctocephali does not differ in trunk spine distribution with respect to P. cetaceum, and that its morphometric differences can be subsumed under the natural variation found within P. cetaceum populations. Therefore, P. arctocephali becomes a junior synonym of P. cetaceum. P. cetaceum was transferred from Corynosoma to Polymorphus due to the absence of genital spines in both sexes. However, adopting the less restrictive definition of genital spines used by several authors, females of P. cetaceum could be considered as bearing genital spines. Species of the genus Andracantha also have genital spines, but specimens of P. cetaceum possess a continuous field of trunk spines, which precludes the assignment of this species to Andracantha. Other generic level characters, as well as ecological data, support clearly the transference of P. cetaceum back to Corynosoma. Therefore, this species becomes Corynosoma cetaceum Johnston & Best, 1942.