Foraging northern bats Eptesicus nilssoni were monitored from a car along a 27 km line transect in southern Sweden every week during a 14 month period by means of a bat detector. The number of bats observed along the transect was highly correlated with air temperature, and no bats were observed at temperatures < 6°C. Hence, feeding was infrequent in April and May as well as in September and October and did not occur at all from November to March. In summer, the bats were observed in forest and farmland, but in spring and autumn most bats were detected along rows of street-lights. By attracting insects, artificial lights apparently provide local patches of food for some species of bats during periods which may be critical for their survival and reproduction.