Adults of Aulacophora nigripennis terminated diapause when transferred from outdoor conditions to a long photoperiod before February but remained in diapause when transferred to a short photoperiod. Both mating and vitellogenesis occurred at either photoperiod in adults examined in February or later. The supercooling point (SCP) of adults was about −6°C in summer and −10 to −11°C in late autumn and winter. Summer adults rapidly decreased the SCP to about −16°C after deprivation of food for 4 days at 25°C, suggesting that the gut contents play an important role in lowering the SCP. Among the cryoprotectants known in insects, only myo-inositol was found accumulated in overwintering adults of A. nigripennis. Female adults accumulated more myo-inositol than males in February. An exposure of overwintering beetles to 25°C caused a rapid decrease in myo-inositol level even at a short photoperiod. This indicated that the reduction in inositol may be independent of the termination of diapause.