We monitored northern bat Eptesicus nilssonii (Keyserling & Blasius, 1839) acoustically along a 27 km road transect at weekly intervals in 1988, 1989, and 1990, and again in 2016 and 2017. The methodology of data collection and the transect were the same throughout, except that the insect-attracting mercury-vapour streetlights along parts of the road were replaced by sodium lights between the two survey periods. Counts along sections of the transect with and without streetlights were analyzed separately. The frequency of bat encounters in unlit sections showed an average decline of 3.0% (± 0.4 SE) per year, corresponding to a reduction of 59% between 1988 and 2017. Sections with streetlights showed an 85% decline over the same period (6.3% ± 0.4 SE per year). The decline represents a real reduction in the abundance of bats rather than an artefact of changed distribution of bats away from roads, following the disuse of insect-attracting mercury-vapour streetlights. Our results conform with another long-term survey of the same species on the Baltic island of Gotland. This decline agrees with predictions based on climate change models, and may also be explained by increased competition from pipistrelle bats (Pipistrellus spp.) expanding from the south. However, the decline may also be due to changed availability of preferred prey (moths), a likely effect of the change in streetlighting. In the 1980´s, E. nilssonii was considered the most common bat in Sweden, but the subsequent decline would rather qualify it for vulnerable or endangered status in the national Red List of Threatened Species.