nvestigating mating systems of species with a cryptic lifestyle often requires a combination of behavioural and genetic data. We used such a combined approach to investigate the mating system of the communal breeding Bechstein's bat (Myotis bechsteinii). Although females of this species are philopatric, gene flow among colonies is high. Gene flow occurs if dispersing males mate within the colony to which they moved. Males could gain local matings by defending resources or females in their breeding habitat. Alternatively, mating may take place at swarming sites, apart from the breeding habitat of the females, where males and females of several colonies meet. Whether or not males defend resources or females in the breeding habitat is of importance for understanding the mating system. Detailed observations of individual foraging and roosting behaviour over 4 yr suggest that males do not defend resources or females to gain matings. Moreover, paternity assignment based on microsatellite data of four complete juvenile cohorts showed that local males fathered less than 25% of the juveniles born in the colony where they settled. Even more striking, none of the males that had immigrated into our study area reproduced with the local females.