Hibernation is an adaptive strategy in bats that facilitates coping with low ambient temperatures (Ta) and scarce food during winter. The decline in metabolic rate (MR) and body temperature (Tb) of a bat during hibernation brings about a significant reduction in nutrients and water use, enhancing the probability of survival (Geiser and Koertner, 2010). However, hibernation is not a constant state of reduced Tb and MR, rather it comprises bouts of torpor interspersed with periods of arousal, when the animal returns to its normothermic Tb and MR (French, 1985). Although, bats arouse for only 5-10% of the time that they hibernate, arousals can account for over 85% of a hibernating bat’s energy expenditure (Wang, 1978). Many different hypotheses attempting to explain periodic arousals have been proposed (Thomas and Geiser, 1997); all relate arousals to processes, such as metabolism and water loss, that vary in the same direction with Tb and Ta. Consequently, they give rise to the same prediction, namely that torpor bout length (TBL) is negatively correlated with Ta and Tb. This correlation alone cannot establish a causal link between arousal and the proposed processes, and, in addition, it introduces a difficulty in distinguishing between the hypotheses. We tested the “water balance” hypothesis, first proposed by Fisher and Manery (1967) that asserts that hibernating animals continuously lose water through evaporation while hibernating, and the ensuing dehydration initiates arousals, during which the animal rehydrates by drinking (Thomas and Geiser, 1997). We tested our hypothesis in groups of a small bat, Kuhl’s pipistrelle (Pipistrellus kuhlii), that were all maintained at constant Ta and constant absolute humidity. We assumed that at constant Ta, bats have similar Tbs and MRs, and that water vapor density at the skin surface is saturated at skin temperature. We manipulated the difference in water vapor density (ew) between the skin surface (es) and the adjacent ambient air (ea) by changing ea, and measured water loss in Kuhl’s pipistrelles in both dry and humid air, at constant Ta. We found that TBL and TEWL were significantly related in hibernating Kuhl’s pipistrelles, independent of MR and/or Ta, supporting the water balance hypothesis, while distinguishing it from other hypotheses that relate processes correlated with MR, Ta, or both. We also found that arousal frequency during hibernation was positively related with the amount of body mass (mb) lost during this period, suggesting that bats with higher rates of TEWL that awoke more frequently lost more mb and thus end hibernation with smaller fat reserves. These results demonstrate the importance of TEWL in survival of overwintering bats. Most, if not all, temperate zone bats in both suborders, the Yangochiroptera and Yinpterochiroptera, are hibernators (Altringham, 2011). Therefore, due to the similarities in the ecological and behavioral characteristics of these animals, the possibility exists that what we found in P. kuhlii, a vespertilionid bat, may occur in other hibernating bat species of different families.