Mumbulla mine is an abandoned mine on the far south coast of NSW. It is notable for its significant population of Eastern Horseshoe Bats Rhinolophus megaphyllus. Bat species and numbers inhabiting the mine were monitored during May, July, October and November 2002 and January, March, April and October 2003. Bat numbers were recorded through the use of an infrared gate and data logger with a maximum number of 999 bats recorded in October 2002 prior to the maternity season, and a minimum of 54 bats recorded in January 2003, during the maternity season. Eastern Horseshoe Bats were identified using an Anabat detector during all monitoring sessions while the Eastern Bent-wing Bat Miniopterus schreibersii was absent during November and January. Sustained activity through the night at the mine entrance in April/May coincides with the mating season, and this observation is consistent with bat swarming behaviour, not previously reported in Australia. Both bat species were recorded flying at the mine entrance throughout year, including winter. Winter activity indicates that surveys at mines, using an Anabat detector, can be productive during the cooler months of the year. Despite the fact that no evidence of breeding was noted during this study, high numbers through winter, peaking just prior to the maternity season and possible mating behaviour in autumn, indicates that the mine should still be regarded as a high priority for management.