The pelvis of the Antillean fruit-eating bat (Brachyphylla cavernarum) displays strong sexual dimorphism. Here, we describe osteological criteria for determining the sex of this bat species. These criteria are applied to fossil deposits from the Blanchard Cave on the island of Marie-Galante (French West Indies), which contains abundant bat remains. This cave is currently occupied seasonally by a colony of Antillean fruit-eating bats as a nursery roost. Sex-ratios established for individuals from different fossil layers demonstrate the site to have been home to a nursery roost as early as 27,000 years ago, and probably going back as far as at least 40,000 years. These nursery roots are shown to alternate with periods where faunal assemblages were accumulated by owls.