Background: Previous descriptions of Corynosoma bullosum (Linstow, 1892) Railliet & Henry, 1907 show that the specimens vary greatly for the proportions of the different structures of the body, measurements of females and males, number of rows of hooks, egg measurements, among others. Here, we redescribe this species from specimens found in the southern elephant seal (Mirounga leonina) feces, from 25 de Mayo/King George Island. We also provide a molecular characterization, in addition to 5.8s and ITS existing sequences.Results: We examined feces collected from 41 elephants in 2016 and 2018. We found 30 adult acanthocephalans in 15 of them (19 females and 11 males). The specimens were identified as belonging to the genus Corynosoma due to the tubular body with an inflated anterior part forming a thorny disk and the posterior part bearing somatic spines on the ventral surface, and genital spines surrounding the genital pore. The morphology of the individuals corresponded to C. bullosum: large size, marked sexual dimorphism (females 13.7-20.0mm and males 9.5-13.6mm in length), and proboscis with 16-18 rows of spines with 11-15 spines per row (8-12 apical and 3-4 basal). The molecular profile of two specimens of C. bullosum was analysed using cytochrome oxidase 1 (COI). Finally, we inferred phylogenetic relationships of the family Polymorphidae using maximum likelihood (ML) and Bayesian inference (BI).Conclusions: We provided an updated morphological redescription for C. bullosum including electron microscopy photographs and molecular data. We also present a summary table showing the great morphological variation of the species. The COI gene sequences revealed that C. bullosum has low genetic variation and that this species is more closely related to Andracantha sigma than to the genus Corynosoma. Our analyses show that it is necessary to perform a phylogenetic study of the family Polymorphidae including all the species.