Workers of Paraponera clavata, a common Neotropical ant, collect both nectar and insect prey. Previous reports show that nectar accounts for up to 90 percent of the ants' food loads, while calculations suggest that nectar contributes only 10 percent of colonies' energy supply. We assessed the trophic source of carbon and nitrogen in adult workers using stable isotope analysis. Carbon in adult workers was largely derived from plant sources. Worker nitrogen isotopic ratios varied significantly among colonies and were enriched compared to prey. Prey nitrogen isotope ratios suggest considerable intercolonial variation in diet, with some colonies collecting prey from lower trophic levels than other colonies. The importance of nectar as a source of metabolic carbon in adult worker biomass, coupled with the high frequency of nectar collection, supports the conclusion that omnivory is a key to supporting this species' biomass in Neotropical wet forests.