Communal night roosting behavior of Myotis lucifugus was studied in southeastern Ontario using low light level television equipment. Adult females roosted in groups of up to 37 in five mortices in two barns. Night roosting only occurred on cool nights (minimum temperature below 12°C) during May and June, a period corresponding to that of gestation. No roosting occurred on nights above 12°C. On nights between 5 and 12°C each bat used the roosts up to three times per night and marked individuals each had a consistent pattern of behavior. On colder nights (below 5°C), all bats entered the roosts early in the evening and stayed until just before dawn. At night, the night roosts were consistently warmer than ambient and day roost temperatures. Communal night roosting thus serves a thermoregulatory role for pregnant female M. lucifugus which must maintain a high body temperature to promote rapid embryo development, a vital factor in the survival of young over their first winter. Individual patterns of roosting activity and the correlation between temperature and roost use likely result from the trade off between the thermoregulatory benefits and commuting costs of night roosting.