Sexual, ontogenic and temporal effects in the diet of Dasyatis longa were evaluated to determine feeding habits and trophic ecology. Numeric indices and the index of relative importance were applied to establish the feeding strategy of the species. Independence of the diet with respect to sex, dry or rainy season and size was evaluated with contingency tables, correspondence analysis and multivariate analysis (MANOVA). The trophic relationships of D. longa (by sex and size intervals) were determined using Levin's niche breadth index and the Pianka's diet overlap index and their significance was determined by null models. The trophic level for each size interval and the species was also calculated. Dasyatis longa showed a narrow niche breadth feeding mainly on shrimps and fishes and its diet was dependent on size, but not on sex or season. Juvenile individuals (class I) fed on shrimps, sub-adults (class II) on fishes and adults (class III) on stomatopods. Significant overlaps between size classes I and II and classes II and III were found. The trophic level shows D. longa to be a secondary or tertiary consumer. Due to considerable fishing pressures on shrimps, the principal prey of D. longa, it will be important to determine their ability to adapt to changes in prey populations.