This study presents results from the first survey of the helminth parasites of fishes in the Ayuquila River, Sierra de Manantlán Biosphere Reserve, in the states of Jalisco and Colima, west central Mexico. Twenty-eight helminth species were collected from 14 freshwater fish species in July 2000 and February 2001. No helminth species collected is exclusive to the Ayuquila River: 11 are allogenic species, mostly Nearctic in origin, and 17 are autogenics. Three introduced species were recovered. Saccocoelioides sogandaresi (Trematoda) was the most prevalent and widespread helminth recovered. The fish helminth fauna of the Ayuquila River is dominated by trematode and nematode species with only a few monogenean and acanthocephalan species. The fauna exhibits considerable overlap with that reported for other freshwater basins in Mexico, and all helminths recovered have been reported previously from Mexico.