The native continental ichthyofauna of Chile has a low richness in species and marked endemism in comparison to the rest of South America. Since the end of the 19th century 26 fish species have been introduced into the Chilean systems. The study covers the trophic ecology of three native and two exotic fish species from the Chillán River. Stomach contents were analyzed and prey items identified at the family level, then compared with the environmental availability by sampling the benthos and using Ivlev’s Electivity Index to determine the degree of election. The relative importance of each prey item was determined and the Morisita Index applied to determine the level of overlap. The results indicate a greater trophic spectrum for introduced rather than for native species. The Relative Importance Index identified Chironomidae as the most important prey item for all species studied. An elevated overlap was observed between the diets of the five species, being greater within the native and introduced species (> 0.9) than between the two groups (< 0.8). These results do not reveal competition, but allow the authors to infer the negative effects of the introduced species to the conservation of native fish.