The mating system of a migratory bat, Miniopterus minor, was studied in coastal Kenya, East Africa. Although this small (6.3 g) cavernicolous bat is not sexually dimorphic in size, aggregations of males secreted a pungent odour during their annual occupation of a particular erosion hollow, which appeared to function as a lek. This site was monopolized, for some two months before mating, by a group of about 30 adult males that were highly site specific for the cave. These bats were heavier than unsuccessful males and, having occupied the lek one year, were more likely to gain access to it in subsequent years. Potential determinants of the mating system are discussed in relation to observations of other miniopterene bats.