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We describe a model which estimates grasshopper body temperature (T-b) by linking energy-flow equations with empirical descriptions of aboveground gradients of air temperature (T-a) and wind speed. The model was tested using restrained grasshopper nymphs; estimated and observed T-b agreed well (r(2) > 0.81). At a rangeland site near Lethbridge, Alberta, Canada (49 degrees 4’N, 112 degrees 48’W), we observed 315 free-living grasshoppers. We recorded the shadow each cast on a horizontal surface, then reconstructed their orientation to the sun by geometric analysis. We used the model to estimate their T-b and the range and frequency of possible T-b within their environment. Modelled T-b exceeded T-a, and was generally lower than the modelled maximum possible T-b, but was well correlated with T-b Of insects on top of the dense layer of vegetation which pervaded the site. This observation suggests that behaviours which elevate T-b are constrained by environmental barriers. T-b exceeded the value expected if insects were located and oriented randomly within their environment (mean difference = 3.95 degrees C, SE = 0.115); this is unequivocal evidence for behavioural thermoregulation. Heuristic simulations using temperature-dependent developmental- and feeding-rate equations for Melanoplus sanguinipes (Fabricius) suggest that thermoregulatory behaviour increased these rates by 30-40% compared with those for insects located and oriented randomly within their environment. During this study, population processes were never inhibited by excess heat; therefore any climatic warming at the experimental site will probably accelerate the phenology of these grasshopper species. Effects at other sites may differ; the model can be applied to test this possibility.