The frillneck lizard Chlamydosaurus kingii is an arboreal lizard that is a conspicuous component of the reptile fauna of the wet—dry tropics of northern Australia during the wet season. During the dry season, however, they are secretive, and a previous study revealed that during this season they remain perched in trees and have field metabolic rates only 28% of the wet season levels. Body temperatures (Tb's) of the lizards were measured by ratio telemetry throughout the day during the wet and dry seasons. The midday Tb's during the wet season high (grand mean = 36.7°C) and typical for heliothermic lizards, but the dry season midday Tb's were significantly lower (grand mean = 32.8°C). Microclimatic data and physical characteristics of the lizards were used in a biophysical model to calculate the operative temperatures (Te) of lizards in the shade, in the sun on a horizontal plane, and normal to the sun at each hour of the day for the two seasons. The Te's revealed the physical possibility for the lizards to achieve much higher Tb's during the dry season than were measured. Thus, the lower Tb's in the dry season represent a shift in preference rather than an inability to attain a high Tb's during the cooler dry season. Inspection of the Tb's and Te's revealed that although the lizards remained cooler in the dry season, they did not thermoregulate at the lowest possible Tb's. During both seasons the lizards basked in the sun early and late in the day, but during the dry season the lizards stopped intensive basking at a Tb °4°C lower than in the wet season. An index of the extent to which the lizards exploit the available thermal environment indicates that they thermoregulate carefully in both seasons. Tb's were also measured in a laboratory thermal gradient during both seasons, and the Tb's selected during the dry season were significantly lower than those selected in the wet season. This suggests that the seasonal shift in thermal preference is acclimatization response or an endogenous seasonal cycle rather than a response to a simple thermal cue. The lower Tb's in the dry season result in a conservation of energy and water during a season when these resources are relatively scarce. However, the fact that the lizards do not thermoregulate at the lowest possible Tb's suggests that the dry season Tb's represent a compromise between conservation of resources and the ability to perform other functions such as escape predators and/or digest food.