Head and body length 6.5 to 8.0cm. Wingspan 36.5 to 45.0cm. Females are larger than males. The last Greater Mouse-eared Bat recorded in the United Kingdom was at least 13 years old. During summer males are solitary or live in small groups, usually roosting in buildings with large roofs.
Females form large nursery colonies in buildings or caves.
Interchange between nursery colonies seems to occur frequently. Through winter the Mouse-eared Bat is generally solitary. During the 1950s and 1960s a colony of 3,000 in a Polish church disappeared, as well as other colonies in the Ardennes of Belgium and Luxembourg. The Greater Mouse-eared Bat was last seen in the UK in 1990. The Greater Mouse-eared Bat is primarily a cave dweller and suffers from extensive disturbance and loss of roosts.