As natural roosts of Big-ear Bats (Corynorhinus townsendii ssp.) are threatened or disappear, artificial roosts offering an acceptable microclimate may become an important component of the conservation strategy for this species. To determine how microclimate affects roosting behavior, I studied the responses of a maternity colony of Townsend's Big-eared Bats (C. t. townsendii) to varying temperature and humidity conditions in an attic roost. I constructed matrices of ambient temperature (T_a) and humidity from data logger recordings. I used a thermal imaging system to image the colony at 4 times on each of 4 days during the lactation period, recording size and location of each cluster or solitary individual. Clustering where it was warmest and apparent vasoconstriction of blood vessels in the ears were used to reduce heat loss at low T_a. Reduced clustering, movement to cooler locations, apparent vasodilation of blood vessels in the ears, and increased resting were used to increase heat dissipation at high T_a. The mechanisms used at both low T_a and high T_a probably also reduced metabolic rate and heat production. Humidity did not appear to affect cluster size and location. These results suggest that large, open-room structures offering a range of T_a and enough space for bats to spread apart would be important characteristics of artificial roosts for maternity colonies of C. t. townsendii.