Bluegills (Leomis macrochirus) collected from discharge (heated) and intake (unheated) areas of a power plant cooling reservoir in South Dakota in 1979 were opportunistic feeders. The number of taxa ingested, evenness values, and meal sizes were generally similar between areas. However, several major differences between areas with respect to the relative contributions of ingested food categories to the diets were observed. Differences in diet composition between areas were explained by the relative abundances of food items as influenced by temperature and substrate for food organisms occurring in the reservoir.