The head and body of the brown long-eared bat measure just 5.5 centimetres, although its wingspan can be as much as 28 centimetres. Although it can live up to 22 years, its average life expectancy is five years. This bat has long, fluffy fur that is light brownish grey on its back and yellowish brown to whitish underneath. Its ears are over half the length of its body.
During the summer, the brown long-eared bat inhabits hollows, bird nesting boxes and buildings in forests, gardens, parks and settlements. During the winter it lives in caves or the basements of buildings. It can be found throughout Eurasia.
Entirely nocturnal, the brown long-eared bat has a rather slow, fluttery flight, usually keeping low over vegetation. It feeds on flying insects or insects on the surfaces of leaves, and it eats its prey either while flying or after alighting temporarily. Not a social animal, a breeding colony comprises 10 to 20 females and a smaller number of males.