As suggested by the bat’s name, its fur is uniformly dark brown and glossy on the back and upper parts, with slightly paler, grayish fur underneath. Little brown bats are insectivores, eating moths, wasps, beetles, gnats, mosquitoes, midges, and mayflies. Since many of their preferred meals are insects with an aquatic life stage, such as mosquitoes, they prefer to roost near water. Often they will catch larger prey with a wingtip, transfer it to a cup formed by their tail, then eat it. Smaller prey are usually just caught in the mouth. They often use the same routes over and over again every night, flying three to six meters high above water or among trees. An adult can sometimes fill its stomach in 15 minutes while the young bats have more difficulty. If they do not catch any food, they will enter a torpor similar to hibernation that day, awakening at night to hunt again.
Females may be sexually mature in the fall after their birth, but males may take a year longer. About half of females and most males breed in their first autumn. They can live up to 33 years, males living longer on average, though the average life span is shorter since about 50% of little brown bats die in their first year.