Activity patterns of bats were investigated at the entrance of a natural karstic cave (Kateřinská cave, Czech Republic). The activity was recorded automatically with a double infrared light barrier allowing discrimination between those bats leaving and those entering the cave. Five periods were defined on the basis of bat flight activity: A) Hibernation period (November–late March), with very low activity; B1) Departure period 1 (late March–mid April), with intensive departure during the first quarter of the night; B2) Departure period 2 (mid April–beginning of June), with emergence activity in the first quarter, and a small number of bats entering the cave in the fourth part of the night. The peak of activity was in the second part of the night. C) Summer period (mid June–mid July), with low activity. D) Autumn period (late July–late October), with very high activity and increasing number of bats entering the cave. The peak of activity was around midnight. All periods showed a non-random temporal distribution and a concentration of flight activity around specific time. There was a positive correlation between the number of bat passes through the entrance and outside ambient temperature and a negative correlation between the number of passes and barometric pressure. Rain had no significant effect on the level of bat activity.