Attic-dwelling male Myotis myotis use different roost sites, but prefer one (or a few) of them and visit others sporadically. Roost-site preference can change, especially during the mating season. The males use their roost sites over years. Mean occupancy of all roost sites shows a maximum during the mating season. Females of different nursery colonies meet at male roost sites as far as 12 km from their colonies. They stay, on average, four days with one male, may join several males during one mating season and may visit their mating sites over years. Males appear to show no special behaviour to attract females but extend the time they are present at the roost site during the mating season. Each male is visited by about seven adult mates, on average. Differences in reproductive success are indicated by the times males were joined by females and by the numbers of mates and copulations. Two typical mating postures, copulation and five different social calls are described. Since females of different colonies meet at the male roost sites, the mating system of Myotis myotis may favour genetic exchange between colonies.