Ictalurid catfishes represent a family of freshwater fishes exclusively North American in distribution. The Balsas catfish, Ictalurus balsanus, is endemic to the Balsas River drainage in central Mexico, which flows to the Pacific Ocean. In this article we present the helminth fauna of this catfish through a survey along its distributional range in the Balsas River basin. Between December 2003 and May 2006, 148 specimens of I. balsanus were collected from 10 localities in the Balsas River drainage. In total, 10 species of helminths were found, including 2 digeneans, 2 cestodes, and 6 nematodes. These parasites correspond to 4 larval forms (Eustrongylides sp., Contracaecum sp., Serpinema trispinosum, and Clinostomum complanatum) and 6 adults (Creptotrema agonostomi, Rhabdochona canadensis, Dichelyne (Dichelyne) mexicanus, Raillietnema kritscheri, Megathylacoides lamothei, and Monticellidae gen. sp.). The results are used to address some biogeographical aspects, particularly the concept of core parasite faunas and parasite faunal exchange in a transitional area.