The food ingestion patterns of the rhesus monkeys (Macaca mulatta) of Group L on Cayo Santiago were quantitatively recorded during two months which coincided with the annual breeding season. Three methods of sampling were used concurrently: interval scan sampling, group location mapping and focal animal observation. These data generated time budgets and specifics about foods selected by six age/sex classes of animals. Overall the animals spent 10.8% of their time engaged in eating. This eating time was distributed as 50.2% on commercial diet/other non-naturally occurring food items and 49.8% on natural vegetation of known or unknown plant part. Distinct diurnal rhythms were observed in overall behaviors and food selection patterns. Soil was part of the daily food intake of all animals. Age/sex class differences in volume of food intake and eating bout interruptions indicated that juveniles may adaptively focus on vegetative food sources during times of social stress. The changing environmental conditions on Cayo Santiago are viewed as presenting the opportunity for future studies of perceptual learning and adaptation by groups and the entire population.