Hexagenia bilineata, a large-bodied mayfly, provided a convenient model for the study of the physiological ecology of the Ephemeroptera. The thorax temperature (Tth) of free-flyingH. bilineatawas significantly different from ambient temperature (Ta), but the gradient between them was only about 1° C, which is probably not biologically significant.H. bilineatais clearly ectothermic, and low wing loading may explain the low Tth.H. bilineataemerges during warm and stable weather, when endothermy would provide little advantage.H. bilineatahad a very high ratio of flight muscle mass to body mass, which is expected to result from selection pressure on flight performance, yetH. bilineatais a poor flyer. This effect may be a result of freedom from weight constraints.