The little brown bat is found in southern Alaska and Canada, across most of the United States, and in some parts of Mexico. It has a total length between 60 and 102 mm and wingspan ranging from 222 to 269 mm. 

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Outside of the hibernation period, little brown bats can be found roosting in buildings, trees, or piles of wood, as well as under rocks, however, the species uses different roost locations during the day and night to deal with ambient temperature changes. Caves or abandoned mines are used by colonies during hibernation periods, which occur between late September and mid-May in northern populations. 

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Little brown bats are insectivorous and feed on flying insects, primarily aquatic. They use echolocation to aid in their hunting, and non-lactating individuals eat about half of their body weight in insects a night (outside of the hibernation period). 

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Hibernating colonies in the species' northern range are highly susceptible to white-nose syndrome, aside from other threats, and the species has seen major decreases in Connecticut, being labeled as Endangered and a Species of Greatest Conservation Need by the Connecticut Wildlife Action Plan. The species is one of the three bat species most affected by the disease, and hibernation sites that used to host thousands of individuals now see between 10-100, if any at all.