The prediction of the composition and structure of fish communities in reservoirs is essential in the management of the fisheries, an activity fundamental in emerging countries where human riverine populations depend on freshwater fish for their animal protein supply. The transformation of the river in a reservoir constitutes an environmental filter for the riverine fish species which, before anything else, must find food resources in the reservoir for their individual maintenance. After the closure of Petit- Saut Dam in 1994, fish were sampled by gillnets in the Sinnamary River in the period 1994-2000 and in the Petit-Saut Reservoir between 1998 and 2000. The stomach contents of the main species were analysed for 6 main food items. Analyses of similarity were conducted in order to compare river and reservoir diets of individual species on one hand and the trophic structure of fish assemblages on the other. Twelve species out of 23 analysed showed significant modifications in their feeding habits. The changes affected species from every feeding guild as established from the river stomach contents data. As a consequence the trophic structure was deeply modified in the reservoir. The abundance of piscivores and herbivores decreased while the relative importance of omnivores and invertivores increased. There was a general tendency for species to increase their food niche breath. The ability of species to modify their feeding habits is shown to be of fundamental importance for the trophic organization of the fish community in the Petit-Saut Reservoir.