Establishing dietary central tendencies and extremes remains an important goal of primate research. While habitat differences and spatial discontinuity are well-documented contributors to dietary variation, other factors including polyspecific associations may significantly impact diet through changes in strata use and/or increased feeding competition. Here, we examine polyspecific association with closely related species as a source of dietary variation in a rain forest primate. Diana monkeys (Cercopithecus diana) in Côte d'Ivoire's Taï Forest frequently participate in mixed-species groups. We use data collected over a 5-year period on 4 Diana monkey groups to examine how association affects group diets. Groups exhibited significant differences in association rates with other guenon species, but this minimally influenced diet when food categories (fruit, invertebrates, leaves) were compared: diet overlap of 4 groups across the study period ranges from 90.8 to 98.1%. Examination of species composition within food categories is more revealing: intergroup dietary overlap decreases to 69.8-79.4% across the study period when comparing species contributing to total frugivory and folivory. These data support earlier findings that Diana monkeys maintain fruit-rich diets by competitively excluding sympatric congeners while highlighting that even selective frugivores such as Diana monkeys may exhibit remarkable dietary flexibility. Our study underscores the fact that broad categorical labels can obscure significant dietary differences.