1. The thermoregulatory capabilities of 18 species of Alaskan bees spanning nearly two orders of magnitude of body mass were measured. Thoracic temperature, measured across the temperature range at which each species forages, was regressed against operative (environmental) temperature to determine bees’ abilities to maintain relatively constant thoracic temperatures across a range of operative temperatures (thermoregulatory performance). 2. Previous studies on insect thermoregulation have compared thoracic temperature with ambient air temperature. Operative temperature, which integrates air temperature, solar radiation and effects of wind, was estimated by measuring the temperature of a fresh, dead bee in the field environment. It is suggested that this is a more accurate measure of the thermal environment experienced by the insect and also allows direct comparisons of insects under different microclimate conditions, such as in sun and shade. 3. Simple regression analysis of species and family means, and analysis of phylogenetically based independent contrasts showed thermoregulatory capability, ability to elevate thoracic temperature, and minimum thoracic temperature necessary for initiating flight all increased with body size. 4. Bumble-bees were better thermoregulators than solitary bees primarily as a consequence of their larger body size. However, their thermoregulatory abilities were slightly, but significantly, better than predicted from body size alone, suggesting an added role of pelage and/or physiology. Large solitary bees were better thermoregulators than small solitary bees apparently as a result of body-size differences, with small bees acting as thermal conformers.