Weight, 13-20 g, rarely to 30 g. Distribution in Texas. Widely distributed over most of the eastern and western parts of Texas, but not yet recorded in the central part of the state. The eastern and western forms are regarded as different subspecies, E. f. fuscus and E. f. pallidus , respectively, and may differ in reproductive habits as discussed below. This species is normally a forest dweller, but it does not hesitate to utilize attics and crevices in buildings, caves, and crevices in rocks for daytime retreats.
Favorite roosts are under the loose bark of dead trees and in cavities of trees.
These bats emerge rather early in the evening and feed among the trees, often following a regular route from one treetop to another and back again. Their flight is relatively slow and direct. Big brown bats are relatively ferocious when captured. They cannot produce a serious wound, however. In winter they migrate or seek hibernation quarters in caves or buildings.