Few studies have focused on the thermal biology of tropical or nocturnal snakes. We recorded preferred body temperatures (T_b) of seven Brown Treesnakes (Boiga irregularis) in the laboratory and compared these to operative temperatures obtained with copper models and T_bs obtained by radiotelemetry from 11 free-ranging snakes on Guam. Operative temperatures on Guam did not vary across refuge types, unless the site received direct solar radiation. In a thermal gradient and on Guam, Brown Treesnakes thermoregulated around two distinct temperature ranges (21.3–24.9°C; 28.1–31.3°C). In the gradient, brown treesnakes exhibited elevated T_b into the higher range only in the evening. On Guam, snakes achieved T_bs in the high range only when direct solar radiation was available during the afternoon, a period when snakes were inactive. Higher mean T_bs on sunny days corresponded with observations of basking behavior.