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Knowledge of temporal variation in bat activity within an area is an essential component in designing bat surveys. Yet few studies have thoroughly examined this aspect of bat biology. We measured bat activity at 3 temporal scales in the southern Arizona Sonoran Desert to develop recommendations for designing bat inventory programs and to provide baseline data on activity within this region. We studied bat temporal activity at 17 water sources from May 1995-August 1996 using mistnets and ultrasonic detectors. Indices of bat activity were higher during the reproductive season (May-Aug) than the nonreproductive season (Sep-Apr). Bat activity did not differ between any of the 2-week time periods within the reproductive season. We found that the pattern of bat activity over the course of the night differed by season and year within seasons. Our results indicate that it is important to determine bat activity in an area before designing bat surveys. Activity measured in a portion of the night may not always reflect total activity. We recommend that when possible, activity should be monitored throughout the night.