Polygyny and promiscuity are the 2 most common mammalian mating systems, whereas monogamy and lek mating are rare. Mammalian mating systems are thought to be influenced by the amount of paternal investment required, defensibility of females, and the stability and size of female groups. With some notable exceptions, male bats typically make no paternal investment and, because of high mobility and broad foraging areas of females, ranges of females often are not defensible. Thus, we would expect most bats to be polygynous or promiscuous; however, mating systems of only about 6% of bat species have been studied. Mating systems of leaf-nosed bats in the family Phyllostomidae have not been well studied, and no species in subfamilies Glossophaginae and Phyllonycterinae, a major radiation of nectar-feeding phyllostomids, have been studied. The buffy flower bat (Erophylla sezekorni) is a phyllonycterine bat endemic to islands of the Greater Antilles. We describe the social structure and mating system of E. sezekorni on Exuma, Bahamas, using capture data, roost observations, and paternity analysis. E. sezekorni roosts in multimale–multifemale groups and female groups are large (50–350 bats) and labile. Males of E. sezekorni aggregate at display areas where they exhibit wing displays and hold territories throughout the 2-month mating season. Mature males also produce garlic-scented supraorbital secretions and ultrasonic display calls. Paternity analysis revealed that females do not mate exclusively with displaying males and that there is limited polygyny and reproductive skew. We also found sexual dimorphism in body mass and condition, with males being both heavier and in better condition than females. Based on large female group size, female group lability, and existence of male mating territories, we hypothesize that E. sezekorni employs a form of promiscuous mating system. More data about female mating behavior are required to test this hypothesis.