The stomach contents of 683 fishes representing 39 species were examined to determine the diet of fish living in fast-flowing waters (rapids) of the Sinnamary River in French Guiana. Aquatic invertebrates were shown to be an important source of food in the rapids biotopes, in contrast to what has previously been observed for slow-flowing waters. Aquatic insects (e.g., the larval stages of Diptera, Ephemeroptera and Trichoptera) make up 55% of the diet of fish living in rapids. Such prey animals were more common in those biotopes than in slow-flowing waters. Twenty-two species of fish, each represented by at least 10 individuals in all samples, were analyzed for their stomach contents. The spectrum of prey items allowed for an estimation of the similarity of feeding habits. These diets were further compared to those inferred for other rivers of the Amazonian and Guianian regions.