Tree-roosting bats are highly social mammals, which often form fission–fusion societies. However, extensive, fine scale data is required to detect and interpret these patterns. We investigated the social structure of Gould’s wattled bats,
Chalinolobus gouldii, roosting in artificial roosts (bat-boxes) over a continuous 18-month period. Network analyses revealed non-random associations among individuals in the roosting population consistent with a temperate zone fission–fusion social structure. Females generally showed stronger associations with roost-mates than did males. Two distinct sub-groups within the larger roosting population were detected. There was also evidence of smaller subunits within these larger roosting groups in spring and summer, with broader mixing at other times of the year. The extensive roost occupancy data collected across all seasons was critical in defining this fine scale, and otherwise cryptic, social structure, and in particular indicating that associations observed during peak activity periods may not be maintained across the year.