Window traps are a common method for standardized insect sampling in forest ecosystems. Operating these traps in an oak forest, we found comparatively high numbers of bats as bycatch. This can be a serious nature conservation issue, but has not been addressed in publications so far. We operated 45 window traps randomly distributed in an oak forest in Austria for four months. The traps were made of two crossed acrylic glass windows, a top cover, and a funnel beneath the windows, directing the catch into a vessel with preservation liquid. In addition to insects, we captured 15 adult bats with these traps, i.e. approx. one bat per 13 trap months: 11 males of Pipistrellus pygmaeus, three males of three other species (Myotis alcathoe, M. bechsteinii and Nyctalus noctula) and a single female of a fifth species (Myotis brandtii). Bats as bycatch in an ecological survey are problematic both from legal and ecological perspective, as the species are strictly protected and as high fatalities might have a negative impact on local populations. Taken sex ratio and species composition into consideration, we assume that the bats are not caught by instance, but that the males might have tried to climb into the vessel searching for new day or mating roosts. Therefore, we recommend simple improvements to the constructive details of window traps (two simple pins or wires across the opening of the jar) to avoid bat bycatch.