Hibernating bats display a wide range of thermoregulatory patterns during hibernation, but to date, the majority of research on the topic has been done on cavernicolous bats. Thus, we measured skin temperature of free-living big brown bats (Eptesicus fuscus) during winter in a building hibernaculum in western Indiana. Torpor bouts were considerably shorter than bouts used by cavernicolous bats, lasting a mean of 3.3 days, whereas arousals lasted on average 5.0 h. There was a significant correlation between roost temperature and length of the euthermic period and a larger proportion of bats aroused on warm days, suggesting big brown bats might be taking advantage of warm nights to forage, although evidence for winter feeding is ambiguous in this population. As has been recorded for other noncavernicolous bats during hibernation, most individuals took advantage of partial passive rewarming during some arousals. We estimate these passive arousals could save bats, on average, 96 mg of fat over the course of a 90-day hibernation period, or enough for 16.5 days of steady-state torpor, and may therefore be crucial to survival through long periods of low temperatures and low insect availability. Overall, our results suggest that hibernation patterns in big brown bats in a building hibernaculum are more similar to those of tree-hibernating bats in the Southern Hemisphere than to cavernicolous bats in North America. Further, it appears that big brown bats display extreme flexibility in hibernating patterns, which is unsurprising given their reputation as a relatively hardy species.