Abstract. Abstract Aim: We investigated the use of food resources throughout the reservoirs and tributaries of the Middle and Lower Tietê River. Methods The samplings were made in July 2000 (dry period) and January 2001 (rainy period). The fish samples were collected using two batteries of gill nets consisting of 8 nets with 10 meters in length and 1.5 meters wide with different mesh sizes, dip net and dragging nets. The stomach contents were dissected, the portion studied was the stomach and the anterior third of the intestines. Food remains found in the stomach were analyzed under a stereoscopic microscope and its identification was performed to the lowest taxonomic category. Results The analysis of stomach contents returned 30 types of food sources, being 21 autochthonous and 8 allochthonous. Most of the diet of the species in the tributaries consists of terrestrial insects, followed by vegetal material, seeds, crustaceans and mud, whereas in the reservoirs the main diet consists of aquatic insects, vegetal material, seeds and scales. The species of fish analyzed were grouped into 13 trophic categories. In both reservoirs and tributaries, most of the diet was carnivorous, whereas the omnivorous diet occurred only in the reservoirs. Species from rivers are more specialized and in reservoirs they are more generalist, making it a better survival strategy. Conclusions Fishes use the coastal areas of the reservoirs and even of the tributaries to feed on superior plants and insects that have terrestrial and aquatic stages. The trophic characteristics exhibited by the fish in the reservoirs depend on the structuring of the community during the colonization process, the characteristics of each reservoir, the introduction of exotic species of fish and alternative food sources.