The northern bat is medium-sized (240-280 mm wingspan) and characterized by its shaggy dark brown fur with yellow tips. Its ears are round and dark as its skin.

Eptesicus nilssonii is common in northern Europe and can also be found beyond the Arctic Circle. If favours habitats such as agricultural areas and woodlands. Summer roosts include crevices in buildings, while in winter Eptesicus nilssonii hibernates in houses, cellars and sometimes mines.

This species forages in open areas and around street lamps, where large insect swarms tend to gather. By undertaking rapid dives, the northern bat is able catch small Diptera, beetles and other insects.

Although it appears to be a sedentary species, ring recoveries have shown that occasionally long distances (100-450 km) have been undertaken.