This study describes the diet composition of 42 rainbow trout, Oncorhynchus mykiss (Walbaum, 1792) (90-480 mm TL), captured in May 1995 in the Urederra River (North of Spain). Rainbow trout of all sizes fed mainly on midge (adults, larvae and pupae), but there were some differences in the feeding habits of small and large fish. We found that as the trout grew larger terrestrial prey became the most important food item. Rainbow trout avoided Leuctridae and showed some preference for Ephemeroptera, Diptera larvae and Nemouridae larvae. Native fish predation and exploitation of the same trophic resources in the Urederra River, could negatively affect native fishes, such as European minnow, Phoxinus phoxinus L., 1758 or brown trout, Salmo trutta L., 1758.