The short-horned lizard Phrynosoma douglassi was studied in a montane habitat (elevation of 2290 m) in the Guadalupe Mountains of Texas. The body temperatures (T(b)) of active lizards were consistently between 35-36°C (grand mean = 35.5°C) in the period from 0900 to 1800 h during the months of May through September. The lizards began their spring activity during April when environmental temperatures were still low. Although T(b)s in April were significantly lower than during May through September, on sunny April days the lizards were able to maintain T(b)s near 30°C even when air temperatures were as low as 1.5°C. P. douglassi at this site thermoregulated very effectively whenever they were not limited by the physical environment.