Prior to arrival of white-nose syndrome, we found bats hibernating in 82 of 119 abandoned mines in northern Michigan. Unoccupied sites typically were short (19 ± 17 m SD) and/or experienced chimney-effect airflow, which led to temperatures near or below freezing (-0.8 ± 2.9 ºC). Overall, occupied sites were more structurally complex, longer (307 ± 865 m), and warmer (5.7 ± 3.0 ºC) than unoccupied mines. Number of bats varied from 1 to > 55,000, although the median was 115. Perimyotis subflavus (Eastern Pipistrelle) and Eptesicus fuscus (Big Brown Bat) accounted for only 0.5% of the total of 244,341 bats that were observed. Ninety percent of hibernating animals were Myotis lucifugus (Little Brown Bat), and almost 10% were M. septentrionalis (Northern Bat). Relative to Little Brown Bats, Northern Bats were more common in the mines of the Upper Peninsula than in hibernacula in the East and Ohio River Valley. Maximum ambient temperature, presence of standing water, and water vapor pressure deficit were potential predictors of the number of Myotis that was present. Seventy-five percent of Northern Bats and 22% of Little Brown Bats roosted alone, rather than cluster with other bats. Little Brown Bats in Michigan were solitary much more often than in the East.