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We compared the intensity of thermal constraints on the activity of the lizard Cophosaurus texanus found in two populations in Big Bend National Park, Texas. The low elevation site was known to be hotter than the high elevation site; we asked whether this coarse-gain environmental difference was relevant to C. texanus. We used hollow-bodied copper models and transect sampling to measure the operative environmental temperatures and relative availability of microhabitats at both sites. Based on laboratory measures of the critical thermal extrema and the range of field-active body temperatures of lizards in each population, we estimated the percent of the environment thermally acceptable to C. texanus. We found that the high elevation site had on average more than 70% of operative environmental temperatures thermally acceptable during midday, whereas the low elevation site had less than 20%. Thus, we concluded that the activity of C. texanus should be more constrained by the thermal environment at the low elevation site. To verify this result, we compared the actual microhabitat use patterns of lizards at each site with a null model of random use of thermally acceptable microhabitats. Predicted preferences more closely matched the observed behavior of lizards from the low elevation site, demonstrating that the activity of lizards at this site was more greatly influenced by the thermal environment than at the high elevation site.