This note reports an unknown trophic interaction between a mammalian herbivore, the capybara (Hydrochaeris hydrochaeris), and the seagrassRuppia maritima (wigeongrass) and compares the feeding behavior of capybaras to other seagrass grazers. Observations were made in Spring 2002 in the Barra Grande, a small, shallow, moderately stratified, bar-built estuary at Ilha Grande, Rio de Janeiro State, southeast Brazil. The activities of the capybaras were investigated and grazing impacts were quantified in situ. The capybaras were observed feeding onR. maritima during the day and aquatic feeding alternated with periods of feeding on land.R. maritima was the only submerged aquatic vegetation to be consumed by the capybaras. The feeding activity of the capybaras on wige ongrass consisted of alternately diving down to theR. maritima then surfacing; the capybaras spent a significantly greater amount of time under water. In the area where the capybaras foraged 18.1% of the seagrass meadow showed recent grazing scars. Vegetation of recently and not recently grazed areas were compared. Grazing scars, which were slightly elongated, were not completely devoid ofR. maritima but presented reduced standing crop: canopy height, shoot density, and shoot, rhizome, and root biomass were reduced in grazed areas. The grazing patterns observed in capybaras resembled those previously reported in the sirenia, mammals that include two seagrass-eating species.