Southwellina hispida Van Cleave, 1925 (Polymorphidae) is an endoparasite of fish-eating birds; the taxon exhibits wide intraspecific variability along its distribution area in Mexico. Specimens of S. hispida were recovered from 12 definitive host species (herons, pelicans, cormorants, and anhingas), as well as from paratenic hosts (cichlid fishes), along the Gulf of Mexico and Pacific Ocean slopes comprising localities in both the Neartic and Neotropical regions of Mexico. There were no significant differences in trunk length and width of the 40 cystacanths recovered from the mesentery of paratenic hosts. However, a more-detailed principal component analysis of 28 relaxed, mature adult females of S. hispida revealed considerable morphological variation among specimens. Sequences of the mitochondrial coding gene cytochrome c oxidase (cox 1) were generated for 22 samples of S. hispida recovered from 12 localities. The genetic divergence estimated among samples was very low, ranging from 0% to 1%. The 22 cox 1 sequences of S. hispida were aligned with 10 other sequences, representing 3 genera of Polymorphidae and forming a data set of 32 taxa with 655 nucleotides. The maximum likelihood tree yielded the 22 sequences of S. hispida plus 1 sample of S. hispida from Hawaii, United States and formed a clade with a high bootstrap support value. The genetic divergence plus the phylogenetic tree indicated that all the samples of S. hispida belong to the same species. The presence of S. hispida in members of 4 families of fish-eating birds (Ardeidae, Anhingidae, Pelecanidae, and Phalacrocoracidae) could be the result of these birds occurring in sympatry and feeding on the same fish species harboring the cystacanths, showing the low host-specificity of this acanthocephalan.