SUMMARY In the flight muscles of European bumblebees, high activities of fructose-1,6-bisphosphatase (FbPase) relative to phosphofructokinase (PFK)have suggested a thermogenic `futile cycle' important for regional endothermy. We find generally low activities of FbPase (0.7-19.7 units g -1 thorax) in North American Bombus species, with the exception of Bombus rufocinctus , where activity (43.1 units g -1 thorax)is comparable with that of European congeners. These data, taken with estimates of maximal rates of heat production by cycling, do not support a significant thermogenic role for the PFK/FbPase cycle. In agreement with earlier studies, both PFK and FbPase activities were found to scale allometrically with body size (allometric exponents -0.18 and -1.33,respectively). The cycle may serve to supplement thermogenesis or amplify glycolytic flux in rest-to-flight transitions, especially in smaller bees.