I measured natural baseline activity and the response of hibernating bats to human presence in a hibernaculum containing ca. 1,300 bats of the species Myotis lucifugus and Myotis septentrionalis. Infrared detectors registered baseline flight movements in the hibernaculum over 62 days in January to March and over 8 days in April, when no observer was present, and they also registered the increase in flight movements following six visits of 1–2 h each to the hibernaculum. Visits to the mine resulted in a dramatic increase in flight activity of bats beginning within 30 min of the visit, peaking 1.0–7.5 h later, and remaining signifi-candy above baseline level for 2.5–8.5 h. These results show that, contrary to previously published studies, hibernating bats are sensitive to nontactile stimuli and arouse and fly following human visits. To avoid increased mortality due to the premature depletion of fat reserves, human visits to hibernacula should be kept to a minimum.