The helminth fauna of Mexican ictalurids was investigated through original surveys and published literature in order to evaluate the biogeography of host-parasite associations of a primarily nearctic host group in a major faunal transition area. In total, 84 specimens of 4 species of ictalurids (Ictalurus balsanus, Ictalurus dugesi, Ictalurus furcatus, and Ictalurus punctatus) were sampled from 7 localities in 5 hydrological drainages. Ten species of adult endohelminths were recovered from this survey, including 4 species of digeneans (Phyllodistomum lacustri, Genarchella tropica, Alloglossidium corti, and Campechetrema sp.), 3 cestodes (Corallobothrium fimbriatum, Megathylacoides giganteum, and Choanoscolex lamothei), and 3 nematodes (Dichelyne mexicanus, Goezia sp., and Rhabdochona sp.). The adult helminth fauna of ictalurids reported in this study is composed of taxa that are typical of ictalurids in other parts of North America, north of Mexico. This core fauna (in a historical biogeographic sense) includes the corallobothriines C. fimbriatum and M. giganteum and the digeneans P. lacustri and A. corti. Two other helminths, C. lamothei and D. mexicanus, may be added to this group in the future. Other helminths occurred sporadically and provide evidence for host-sharing (ecological host extensions), but we were unable to identify any valid cases of host-switching from more distantly related hosts. The helminth fauna provides evidence that the ictalurids developed their characteristic helminth fauna without neotropical influences and that the phylogenetic affinities of the ictalurids do not appear to be with any neotropical siluriform.