Compared to other microbats, the big brown bat is relatively large, weighing 15–26 g (0.53–0.92 oz) and possessing a wingspan of 32.5–35 cm (12.8–13.8 in).

They are nocturnal, foraging for prey at night and roosting in sheltered areas during the day such as caves, tunnels, tree cavities, and human structures.

While some other bat species in its range have experienced dramatic population declines due to the fungal disease white-nose syndrome, the big brown bat is relatively resistant to the effects of the disease, and some populations have even increased since the syndrome arrived in North America.

Palisot described the species based on specimens in the Philadelphia Museum of Charles Willson Peale, an American naturalist, giving it the name Vespertila fuscus.

[6][7] The genus name, Eptesicus, is likely derived from the Greek words ptetikos ("able to fly") or petomai ("house flier"), and the species name "fuscus" is Latin in origin, meaning "brown".

[8] The big brown bat is the type species for the genus Eptesicus, which was established in 1820 by French-American naturalist Constantine Samuel Rafinesque.

The big brown bat belongs to the serotinus group, which is defined by having a large, elongate skull, flat braincase, and a long snout.

As the big brown bat is such a widespread species, it has regional variation in its diet, though it is generally considered a beetle specialist.

In British Columbia, large proportions of caddisflies are consumed, with flies as a secondary prey source.

[34] A study in Ontario compared fecal samples of big brown bats with the abundance of insects collected in passive traps.

It will utilize a wide variety of structures for roosts, including mines, caves, tunnels, buildings, bat boxes, tree cavities, storm drains, wood piles, and rock crevices.

[34] In the eastern United States, twins are commonly born sometime between May and July; in western North America, females give birth to only one pup each year.

[29] Big brown bats enter into hibernation around November, often in a location less than 80 km (50 mi) away from their summer roosts.

[45][47] While some big brown bats hibernate in subterranean locations such as caves and underground mines,[29] most can be found in warm man-made structures.

[48][49] Big brown bats tolerate cold weather fairly well,[46] although they can be negatively affected by major changes in temperature.

[45] It is fairly common for some hibernating big brown bats to awaken temporarily and seek warmer shelter, locate water, and even mate.

Insect ectoparasites include Basilia (flies), Cimex (true bugs), and Myodopsylla (fleas).

Several mites are ectoparasites as well, including Acanthopthirius, Cheletonella, Euschoengastia, Leptotrombidium, Macronyssus, Neospeleognathopsis, Neotrombicula, Olabidocarpus, Ornithodoros, Parasecia, Perissopalla, and Spinturnix.

[55] In one study, only 10% of big brown bats were shedding the rabies virus through their saliva before exhibiting clinical symptoms of the disease; symptoms of rabies in big brown bats include acute weight loss, paralysis, ataxia (inability to coordinate muscle movement), paresis (weakness of voluntary movement), and unusual vocalizations.

It has also been documented in the Caribbean in both the Greater and Lesser Antilles, including Cuba, Hispaniola, Dominica, Barbados, and the Bahamas.

Its extensive fossil record is known from more than thirty sites, including fourteen US states, Puerto Rico, Mexico, and the Bahamas.

It meets the criteria for this designation because it has a wide geographic distribution, a large population size, occurrence in protected areas, and tolerance to habitat modification by humans.

Even in caves harboring Pseudogymnoascus destructans, the fungus that causes white-nose syndrome, big brown bats maintain normal torpor patterns.

Unlike in other species more affected by white-nose syndrome, big brown bats are able to retain more of their body fat throughout hibernation.

In fact, some regions of the eastern United States have seen an increase in big brown bat populations since the arrival of white-nose syndrome.

[59] Big brown bats can be of concern to public health as a rabies vector, as they commonly roost in buildings and thus have a higher chance of encountering humans.