Temperature influences ectotherm fitness by affecting physiological performance. Terrestrial reptiles behaviourally regulate their body temperature by selecting particular microhabitats or activity periods. In temperate climates, thermal constraints make precise thermoregulation costly. Theoretical models of thermoregulation predict that species in cool environments should exhibit lower optimal temperature for performance and lower thermal preferences to minimize thermoregulatory costs. Empirical data in support of this prediction remain equivocal because several species maintain high and constant body temperatures, even in cool environments. We studied two largely sympatric colubrid snakes, Hierophis viridiflavus and Zamenis longissimus that share numerous morphological and ecological similarities, but differ markedly in thermal preference. Our objective was to quantify their thermoregulatory strategies in the field to determine how thermal preferences translate in habitat use and performance gain. The thermophilic species, H. viridiflavus, selected open microhabitats, whereas Z. longissimus, which prefers cooler temperatures, used a greater diversity of microhabitats. The two species differed markedly in their exposure levels. Hierophis viridiflavus was constrained to shuttle between sun and shade to maintain preferred body temperatures rendering it very exposed, while covered microhabitats were usually thermally compatible with the requirements of Z. longissimus. High exposure was apparently counterbalanced by higher locomotor performances in H. viridiflavus. The divergence in thermal ecology between Z. longissimus and H. viridiflavus likely reflects different trade-offs between energy gain and predator avoidance.