The relative importance of plants and insects as sources of protein through the year was evaluated in 2 bat species in a seasonal forest in the neotropics using stable-isotope analysis of carbon and nitrogen. Artibeus jamaicensis, a frugivore, met almost all of its protein requirements from plants with no seasonal or sexual variation. In contrast, Glossophaga soricina, a nectarivore, relied mostly on insects, but in females, plants and insects were equally important in the early rainy season and in the mid dry season. Evidence of changes in food origin of protein associated with reproductive activity was not detected in either species.