Big brown bats are medium brown in color, with darker facial features and ears. Their small ears and eyes sit upon a blunt face. This bat can be seen in urban and suburban environments of mixed agricultural use. As generalists, the species can be at home among timberline meadows to lowland deserts, and abundant in deciduous forest areas. Big brown bats will roost in artificial structures, including buildings, bridges, and bat houses. When selecting for hibernation roosts, this species can tolerate hibernating in areas that experience very cold temperatures. Big brown bats have a preference for munching on beetles but will eat a variety of insects. Females can consume up to their body weight in insects in a single night. In the west half of this species' range, females have been found to produce a single pup (baby bat), whereas twins are more frequent in the east.