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The thermal quality of diurnal refuges is important to the performance and survival of nocturnal reptiles. We studied refuge use on both slopes of an east-west-oriented hill by the thigmothermic gecko Hornonota darwini, the southernmost-distributed nocturnal lizard in the world, in the vicinity of Bariloche, Rio Negro, in the Patagonia of Argentina. Because of the harsh climatic conditions in Patagonia, suitable refuges are limited, and retreat-site use is important for these geckos. Honionota darwini used refuges significantly more frequently on the warmer western slope in our study site. Geckos on the western slope used those refuges with higher temperatures regardless of size and thickness of rocks that acted as retreats. We tested whether refuge temperature affected locomotor performance of these lizards. Performance experiments showed that maximum sprint speed was affected by the temperature of the refuges. Refuges at 22.5°C allowed lizards to achieve their fastest sprint performance. Unexpectedly, sprint performance of lizards that used refuges with temperatures >32°C was the lowest among all tested refuge temperatures (18°, 22.5°, 27.5°, and 33°C). Our data illustrate the importance of the thermal quality of refuges for reptiles living in extreme environments. [ABSTRACT FROM AUTHOR]