The locomotor activity rhythm was examined under (1) constant darkness at 25C and (2) light-dark cycles (LD 12:12) at various temperatures ranging between 15 and 25C in adults of Gekko japonicus, a nocturnal lizard living in the temperate zone. The rhythm free-ran in constant darkness with a period of approximately 24 h, and thus the rhythm is controlled by a circadian clock. Gekko japonicus was more active at higher temperatures. Although there was a possibility that the activity distribution in photophase and scotophase in G. japonicus was affected by temperature, it showed nocturnal activity with a peak at 1–2 h after lights-off under LD 12:12 at all temperatures examined.