Barbasteiles are commonly recorded in the vicinity of their wintering underground sites outside the hibernation season. It is uncertain why barbastelles visit hibernacula at this time of the year. Possible functions include feeding, resting, rearing of young, mating or stop-over sites during migration. To determine which of these functions are important I carried out observations at six large (max. 1,870 individuals) barbastelle hibernacula in SW Poland. To characterize daily and seasonal patterns of shelter use behaviour surveys were carried out at regular intervals throughout 2003 to 2005. Bats were mist-netted (73 nights) at hibernacula entrances and their age, sex and the development of caudae epidydimides in the males were determined. Their behaviour was sampled by recording (315 hours) their calls with a time-expansion bat detector in combination with direct observation and infrared filming. Hibernacula were not used as colony roosts, foraging places, day resting places or temporary night shelters. The hibernacula served primarily as mating sites. Both females and sexually active males (with fully developed caudae epidydimides) were seen in the highest numbers in August/September, when display flights, chases of individuals, low frequency social calling and copulations were observed. As the number of bats visiting hibernacula in the mating period was much higher than that observed in winter, the underground shelters appear to be at least as important for them in the mating period as in the winter. Barbastelle hibernation sites should therefore be protected from the beginning of mating period (August) till the end of hibernation.