Noting that captive hibernating big brown bats (Eptesicus fuscus) aroused from bouts of torpor at precise times of the day (17:00–19:00), Twente and Twente (J. W. Twente and J. Twente. 1987. Can. J. Zool. 65: 1668–1674) hypothesized the existence of a temperature-compensated, non-free-running biological alarm clock that regulated the timing of arousals in bats. I tested this hypothesis in a natural hibernaculum used by little brown bats (Myotis lucifugus), northern long-eared bats (Myotis septentrionalis), and big brown bats (E. fuscus) by recording the timing of periods of activity with an ultrasonic detector and an electronic datalogger. Although there was significantly (2.5–4.0 times) more activity during the night than during the day, the pattern of activity did not conform with that observed in the laboratory. Bats were found to be active at all times of the day and there was little evidence of a concentration of activity around the period 17:00–19:00 as observed in the laboratory. Over three measurement periods in early winter and midwinter, the mean activity times were 18:54 ± 4.5 h, 23:48 ± 5.0 h, and 23:18 ± 5.2 h. The high variation around the mean times indicates that bats aroused and were active at most times of the night. If a biological alarm clock exists in bats, it is only weakly expressed under natural conditions.