I used radio-tracking and roost closures to test five hypotheses which explain communal roosting behaviour by the Big Brown Bat, Eptesicus fuscus. These include, a limit to the number of available sites, access to preferred sites where reproductive success is enhanced, avoidance of ambush predators, the two-strategies hypothesis, and reducing commuting distances to foraging areas. Site tenacity, short movement distances and a tendency towards decreased young production with eviction were consistent with the access to preferred sites hypothesis. Although departures were significantly clumped in time, there was no threshold colony size which suggests that ambush predation is only a secondary reason for communal roosting. I tenatively rejected the two strategies hypothesis on the basis of no evidence for following behaviour, a dominance hierarchy or constant cluster composition. Foraging parameters although consistent with minimizing commuting costs, were also consistent with an opportunistic strategy which does not explain communal roosting.