Reptiles possess highly developed behavioural thermoregulatory mechanisms. In tortoises (Agrionemys horsfieldi GRAY) daily rhythms of locomotor activity and preferred temperature were studied under an artificial 12h/12h light‐dark‐cycle in a thermal gradient from 20 to 40°C and under constant ambient temperature. All animals developed a clear daily rhythm of locomotor activity over the whole year. In contrast a daily rhythm of preferred temperature was only found in summer time. These results are discussed under evolutionary aspects.