(Uploaded by Plazi for the Bat Literature Project) During 30 years of observations carried out in over 1,000 underground and semi-underground bat hibernation sites in Estonia, air temperature was measured near sleeping bats. In 1978, the 'lower critical temperature of hibernation' (LCT) was determined for two boreal bat species, Plecotus auritus and Eptesicus nilssonii. The LCT was defined as the mean value calculated on the basis of temperatures at which animals of a certain bat species awoke as a result of temperature decrease at their sleeping sites. For the first species, the LCT was -2.24°C, while for the second it was -3.13°C. During the past 20 years, we have registered new records of minimum hibernation temperatures for the following four sedentary bat species: Eptesicus nilssonii (-5.3°C), Myotis dasycneme (-2.5°C), Myotis brandtii (-2.5°C) and Myotis nattereri (-1.0°C). The highest temperatures (up to +10.5°C) were recorded near bats sleeping in cellar-type shelters in October and November. As a result of research, we concluded that the upper limit of 'hibernation temperatures' in boreal bats is variable depending both on the species and the site. Based on temperature measurements and observations of bat activity during the year, we propose +8°C to mark the upper temperature limit of hibernation for seven bat species under study.