The daily rhythm of behavioral thermoregulation in sleepy lizards (Tiliqua rugosa) was studied in laboratory thermal gradients under a 12L∶12D photoperiod in autumn and spring. In both seasons, lizards showed distinctive daily rhythms of behavioral thermoregulation, selecting their highest body temperatures during the late photophase and lowest ones during the late scotophase or early photophase. When lizards were subjected to a period of constant darkness at both seasons, this rhythm persisted and free-ran with a period of 24.7 h, indicating that it is an endogenous circadian rhythm. Furthermore, the expression of this circadian rhythm varies seasonally in the amplitude of its free-running rhythm and in the phase at which the minimum T_b is selected during 12L∶12D. The seasonal variation in the expression of the circadian rhythm of behavioural thermoregulation in Tiliqua rugosa lizards is likely to reflect seasonal changes in the neural regulation of this rhythm.