We describe the diet of Oligosarcus hepsetus, in the Santa Virgínia Unit of the Serra do Mar State Park. The Paraibuna and Grande rivers in the basin of the Paraíba do Sul River were sampled monthly from January to December 2004. The Alimentary Preference Degree and the frequency of occurrence indices were used to analyze the food items. The diets of Oligosarcus hepsetus in the two localities sampled were very similar, and reinforced the importance of the streamside forests in establishing and maintaining biotic and abiotic conditions in these environments. The species had a carnivorous diet that differed with ontogeny: smaller individuals were principally insectivorous and larger ones ichthyophagous. Keywords