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Reptiles in desert environments depend on habitat thermal quality to regulate their body temperature and perform biological activities. Understanding thermoregulation with respect to habitat thermal quality is critical for accurate predictions of species responses to climate change. We evaluated thermoregulation in Goode’s horned lizard, Phrynosoma goodei, and measured habitat thermal quality at the Reserva de la Biosfera El Pinacate y Gran Desierto de Altar, Sonora, Mexico, during the hottest season of the year. We found that field-active body temperature averaged 38.1 +/- 0.38 degrees C, preferred body temperature in laboratory averaged 34.9 +/- 0.18 degrees C and preferred body temperature range was 32.5-37.3 degrees C. Operative temperature (i.e. environmental temperature available to the lizards) averaged 43.0 +/- 0.07 degrees C, with maximum temperature being near 70 degrees C, and 62.9% of operative temperatures were above preferred body temperature range of P. goodei. Microhabitat thermal quality occupied by the lizards was high in the morning (7:00-10:30) and afternoon (5:50-dusk). We found that despite strong thermal constraints P. goodei was highly accurate and efficient in regulating its body temperature and that it presented a bimodal thermoregulatory pattern, being active in the mornings and in the evenings in order to avoid high mid-day environmental temperatures. Despite its thermoregulatory ability, P. goodei may be vulnerable to climate warming.