Bats are among the most ecologically diverse mammalian orders. Most species live in groups for at least a portion of their life cycle and behavioural evidence suggests that individuals of many species live within complex nonrandomly assorting societies. However, rigorous quantitative characterizations of bat societies have been rare because of the difficulties inherent in studying these highly mobile, small, nocturnal animals. Here we use an automated monitoring system (PIT tags), telemetry, and recently developed analytical techniques to investigate the social organization (size, sexual composition, and spatiotemporal cohesion) and social structure (pattern of social interactions and relationships among individuals) of a colony of free-living northern long-eared bats, Myotis septentrionalis (Trouessart, 1897). Cluster analysis of HWI (half-weight association index) for all pairs and permutation tests indicate that colonies consist of multiple, nonrandomly assorting subgroups. A plot of the temporal persistence of relationships (standardized lagged association rate) showed that roosting groups dissociate over periods of approximately 10 days after which subsets of individuals remain associated throughout the summer roosting season. A model representing a two-levelled social structure of long-term (whole summer) and short-term (up to 10 days) acquaintances best fit the lagged association rate. Subgroups were most cohesive during the lactation period, but we found no evidence for the effects of minimum nightly temperature on subgroup cohesion.