Bats were marked and monitored on Barro Colorado Island, Panama, to study seasonal and annual variation in distribution, abundance, and natural history from 1975 through 1980. Data gathered advances our knowledge about flocking; abundance; feeding strategies; social behavior; species richness; population structure and stability; age and sex ratios; life expectancy and longevity; nightly, seasonal, and annual movements; synchrony within and between species in reproductive activity; timing of reproductive cycles; survival and dispersal of recruits; intra-and inter-specific relationships; and day and night roost selection. Barro Colorado Island (BCI) harbors large populations of bats that feed on the fruit of canopy trees, especially figs. These trees are abundant, and the individual asynchrony of their fruiting rhythms results in a fairly uniform abundance of fruit. When figs are scarce, a variety of other fruits is available to replace them. This relatively dependable food supply attracts a remarkably rich guild of bats. Although we marked all bats caught, we tried to maximize the number of Artibeus jamaicensis netted, because it is abundant (2/3 of the total catch of bats on BCI), easily captured by conventional means (mist nets set at ground level), and responds well to handling and marking. An average Artibeus jamaicensis is a 45 g frugivore that eats roughly its weight in fruit every night. These bats prefer figs and often seek them out even when other types of fruit they might eat are far more abundant. They commute several hundred meters to feeding trees on the average, feeding on fruit from one to four trees each night, and returning to a single fruiting tree an average of four nights in succession. The bats tend to fly farther when fewer fig trees are bearing ripe fruit, and they feed from fewer trees, on the average, when the moon is nearly full. These bats, like their congeners, do not feed in the fruiting tree itself. Instead, they select a fruit and carry it to a feeding roost typically about 100 m away before eating it. We utilized radio telemetry to assess feeding rates from the number of ?feeding passes??transits between fruit tree and feeding roost. Bats are often netted while carrying fruit, revealing their diet. Feces also reveal dietary information. Adult female A. jamaicensis live in harems of three to 30 individuals with a single adult male. On BCI the harem groups roost during the day in hollow trees. There is presumably a large population of surplus males that roost together with nonadults of both sexes in foliage. Females commute an average of 600 m from their day roosts to feeding sites, and harem males travel less than 300 m. Twice a year most females give birth to a single young, once in March or April, and again in July or August; active gestation averages about 19 weeks. Juveniles are first netted when they are about ten weeks old, and females usually first bear young in March or April following their year of birth.