Several species of vespertilionid bat gather at underground sites in temperate zones during late summer and early autumn for an activity called swarming. Up to many hundreds of bats may arrive and depart during a night. Bat activity was recorded automatically with a frequency division logging system at a swarming site in south-east England for 415 nights between 1997 and 2001. In all 5 years, activity was highest between the beginning of August and the end of October (the swarming period), with a peak in September. Activity varied markedly from night to night and was affected by rainfall (which significantly suppressed swarming activity), and residual maximum ambient temperature (with which activity was positively correlated). Moon phase had no detectable influence on swarming activity. Activity was low in the first few hours after sunset of each night during the swarming period indicating that few bats emerged and that there was low daytime occupancy of the site. Activity increased to a peak between 6 and 7 h post-sunset consistent with a large number of bats arriving after the first evening foraging bout. Activity then decreased gradually up to dawn as these bats departed again. Logged activity was positively correlated with the number of bats caught by harp-trap, confirming that loggers are a reliable alternative to catching when monitoring a swarming site.