Simultaneous measures of oxygen consumption and evaporative water loss (EWL) were made in two species of temperate-zone vespertilionid bat (Plecotus auritus and Myotis daubentoni; mean body mass 9.12 and 10.12g, respectively) at ambient temperatures (Ta) of 5, 15 and 25°C and variable vapour pressure deficit. EWL was directly dependent on vapour pressure deficit and oxygen consumption and inversely dependent on Ta. EWL was significantly greater in P. auritus than in M. daubentoni. A model for EWL in P. auritus under a variety of environmental conditions (5–25°C and 20–80% relative humidity) suggested that EWL from bats in shallow summer torpor will be lowest at low Ta, and that, except at low (> 50%) relative humidity, EWL from euthermic bats will be lowest at high Ta. At low relative humidity (< 20%), resting bats could lose over 30% of body mass per day (24 h) through evaporation. At high Ta (> 25°C), EWL from euthermic bats could be over 65% lower at high (> 80%) compared to low (< 20%) relative humidity. In bats in shallow summer torpor at low (5°C) Ta the equivalent saving was > 96%. At low relative humidity predicted EWL from bats in shallow summer torpor was 34 to 81% of that from euthermic bats, and at low Ta and high relative humidity was only 2%. In the wild, M. daubentoni has freer access to drinking water than does P. auritus and yet EWL at rest was higher in the latter species. We suggest that post-prandial dumping of urinary water by M. daubentoni leads to a limit in the amount of body water available to this species to cover evaporative losses once within the day roost, which in turn has led to an adaptation of physiology towards the minimization of EWL when at rest.