Bearded sakis (genus Chiropotes) are among the most highly specialized primate seed predators. However, long-term studies of the genus in continuous forests, with a full community of sympatric primates, are rare. Here I present data on monthly variation in the diet of Chiropotes sagulatus from a long-term study in a continuous forest in Guyana. Bearded sakis had an extremely diverse diet, exploiting more than 175 species of plants. Consistent with their highly specialized dental morphology for seed eating, seeds made up 75% of the annual diet. Sakis exploited a wide variety of mechanically protected fruits and often exploited the same plant species for more than 3 months. They consumed a high percentage of seeds in all months and seed consumption was significantly correlated with fruit abundance. When fruit became scarcer, sakis consumed a higher percentage of non-seed food items, including insects, mature fruit, and flowers. Insects were especially important during the leanest months, making up almost 40% of feeding time. Bearded saki dietary diversity (in terms of plant species) showed little variability across months. These results confirm sakis to be highly specialized seed predators. Sakis preferentially consume seeds when they are available. However, when seeds become scarce, sakis become generalists, supplementing their diet with mature fruit, insects, and flowers. The ability of bearded sakis to consume a diversity of highly abundant plant species, fruit in several stages of maturity, and a variety of different types of resources buffers them from the detrimental effects of resource scarcity. Am. J. Primatol. 75:568-580, 2013. © 2013 Wiley Periodicals, Inc.