Subterranean swarming of bats takes place in the autumn and it is presumed that its main function is to facilitate mating. However, Plecotus auritus (brown long-eared bat) swarms twice a year, in spring (in March and April), and in autumn (from the end of August to October). The premise that both the autumn and spring swarming events have a mating function was tested by measuring the vocalisation activity and reproductive state of males at two subterranean swarming sites and at several maternity colony roosts in southwestern Poland. Vocal activity, as defined by the number of social calls emitted by swarming bats, was about ten times higher in the spring than in the autumn. The bats also emitted a wider range of call types in spring. From 45 to 100% of males examined in spring had distended caudae epididymides. Enlarged and distended epididymes contain spermatozoa and indicate that males are still able to copulate in spring. There was a significant positive correlation between the proportion of males with distended caudae epididymides and the vocal activity of swarming bats. This indicates that swarming behaviour plays a role in mating and that the mating season extends from autumn to spring. For the first time, a biphasic pattern of active mating behaviour has been observed in a European bat species. Mating occurs during swarming in autumn and spring and is accompanied by vocal advertisement. We suggest that the low number of females in spring increases competition between males and significantly increases the amount and diversity of vocal activity.