The diets of the two Amazonian brocket deer (the red brocket Mazama americana and the grey brocket Mamma gouazoubira) were studied in French Guiana through the examination of stomach contents. The two species are mainly frugivore-granivores, even during the annual period of fruit scarcity (56% of the annual diet in red brockets and 68% in grey brockets). Both rely heavily on fruits and seeds, but overcome the fall in fruit availability by eating significant quantities of fibres, leaves and flowers, particularly the red brocket. Seasonal variation in quantity consumed was observed only for gravid females, which suggests that reproduction may be dependent on the fruiting season. Both species took a wide variety of plants (respectively a total of 79 and 107 species, and 8.4 and 8.9 species per stomach on average). They can be considered non-selective feeders, choosing fruits above all according to their abundance, their size, and perhaps their softness. Both species are seed eaters, destroying nearly all seeds ingested. The two species are potential competitors, but their diet competition may be reduced by their large difference in body size and somewhat different ecological niches. These deer are among the most generalized frugivores of the Guianan forest, thus reducing their food competition with other frugivores.