The occurrence of 6 species of helminth parasites (Clinostomum complanatum, Diplostomidae gen. sp., Posthodiplostomum minimum, Bothriocephalus acheilognathi, Glossocercus sp., Contracaecum sp.) is reported from 7 species of freshwater fishes (Chirostoma jordani, Astyanax mexicanus, Herichthys labridens, Oreochromis niloticus niloticus, Abramis brama, Cyprinus carpio carpio, and Poeciliopsis gracilis) in Metztitlán Lake (Laguna de Metztitlán), Reserva de la Biosfera Barranca de Metztitlán, Hidalgo, Mexico. Fish were collected between July 2002 and June 2003. The helminth fauna of H. labridens, an endangered species of Cichlidae, is described for the first time. Migratory birds of the families Ardeidae and Phalacrocoracidae appear to play an important role in the helminth species composition of fishes from Metztitlán Lake. All helminth species represent new records for this federally protected area of Hidalgo.