(Uploaded by Plazi for the Bat Literature Project) This study aims to identify which tree species bats use as maternity roosts in woodland in southern Norway and, to assess the effectiveness of bat boxes as potential mitigating measures if roosts are lost to forestry or other factors. Radio telemetry was used to find roosts in summer during late pregnancy and through the lactation period by tagging two common species; Daubenton's bats Myotis daubentonii (Kuhl 1817) and soprano pipistrelles Pipistrellus pygmaeus (Leach 1825). In addition, tree roosts were searched using ultrasound detectors at three sites. A total of 35 roosts in natural crevices were found using radio telemetry and through searches with ultrasound detectors and 31 of these roosts were in hollow trees, the majority in aspens Populus tremula L. (n = 30). Some tree roosts were even used by the northern bat Eptesicus nilssonii (Keyserling & Blasius 1839) which is considered a house-dwelling species in Norway. Aspens have some qualities preferred by bats and were both warmer and safer than other hollow tree species in this study area. Bat boxes are used as maternity roosts mainly by soprano pipistrelles at these latitudes, thus at present, there is no suitable alternative to hollow trees that would satisfy all tree-dwelling bat species.