This paper reports a study of the visitation of bats to Great Litworowa Cave (1,907 m a.s.l., Tatra Mts., southern Poland) during summer and autumn 1999–2005. A total 5,608 bats representing 11 of Poland's 25 species were captured. Myotis mystacinus predominated. Its activity at all times was high. However, the species composition and number of other bats changed seasonally. Swarming activity lasted between July and November, peaking in late July and August. Nightly activity peaked between 22.00 and 02.00 hrs and then gradually decreased toward dawn. Strong male bias was observed. Myotis mystacinus showed regular changes in sex and age ratios. Rare species such as M. bechsteinii, M. emarginatus, and Vespertilio murinus were recorded. This cave is the highest locality of M. bechsteinii and M. brandtii in Europe. Some behavioural observations including copulation, drinking and daylight activity were recorded. The role of swarming activity is discussed in light of the findings. Some observations suggest that this activity of bats is connected with mating; some other observations provide evidence that the function of swarming is also to facilitate the location of mates and/or to assess suitable hibernacula.