We studied occupation and activity at a hibernaculum and a maternity cave used by the endangered Ozark big-eared bat (Corynorhinus townsendii ingens). Big-eared bats remained active throughout winter, with many individuals leaving and entering the hibernaculum during the first 2 h after sunset; however, no fresh guano was found on cave floors during winter. Big-eared bats used caves with the coldest internal temperatures in the region as hibernacula and also moved among caves throughout winter. The breakup of hibernating colonies in spring and the subsequent buildup of maternity colonies by adult females were gradual. Nightly activity patterns at the maternity cave changed throughout summer relative to parturition and maturation of young. Females returned to the maternity cave more frequently when their young were totally dependent on them for nourishment. As the young began foraging, females returned only once during the night, and eventually adults did not return to the cave until morning. Ozark big-eared bats emerged shortly after sunset, regardless of brightness at twilight or rainfall, but they emerged at a later time when the young could not fly.