This is a northern species of wide distribution and may be the most abundant bat in the United States. It ranges from the coast of Labrador across the Northwest Territories and central Yukon to Alaska. In the Untied States it is known from much of Alaska and all the northern states, ranging southward in the mountains to Georgia and southern California.

In summer the little brown bat inhabits trees, bat houses, and buildings, usually choosing a hot attic, where nursery colonies of hundreds and even thousands form. Less frequently colonies form beneath tar paper, siding, shingles, or other similar sheltered spots,. In the west colonies have been found beneath bridges and in caves. Single males have been found in attics, behind shutters under bark, in rock crevices, behind siding and under shingles. Groups of males occasionally occur in caves.

In a summer colony the bats remain alert on warm days. In any large group a few are usually active, crawling about over the others and bickering. A colony can be heard as far away as 100 feet. Toward evening the bats become more active until a constant squeaking can be heard as they gather about the exits. They begin to emerge at late dusk. When there is water nearby, most of the bats head at once for the water where they begin feeding a few feet above the surface as they continue to disperse.

Little browns breed in the fall. They establish maternity colonies in the attics of buildings and in similar warm, dark retreats. Births occur from May to July, depending on the state. A single young is born each year.

Populations are stable in the Untied States at this point, but these bats are highly susceptible to pesticide poisoning and habitat destruction. Bat houses may be one important way to insure the bats survival.