A maternity colony of the big brown bat (Eptesicus fuscus) was observed from 21 May 1992 to 12 October 1992. The colony consisted of approximately 110 females at the beginning of the study, and about 180 bats following birth of the young. Bats were observed at emergence to gather data on factors affecting initial emergence: light intensity, temperature, relative humidity, barometric pressure, wind speed, cloud cover, and moon phase. Light intensity and cloud cover apparently are the factors most affecting the variation between initial emergence and sunset.