The incidence of pupal diapause in the cabbage white butterfly, Pieris rapae crucivora, fed on artificial diet was investigated under various photoperiodic conditions. The Tokyo population showed a typical long-day response curve with 50% incidence daylength of 12 hr 10 min at 20°C, while a high temperature of 25°C prevented the diapause incidence even under short photoperiods. When the 5th instar larvae which had been exposed to the short photoperiod were transferred to the long photoperiod, their diapause was prevented. In this case, the adult development was delayed in proportion to the intercal between the 4th molt and the transference to long photoperiod. One hour light-break at different times in the scotophase of short-day cycle reduced the diapause incidence. Especially, when the light-break was timed from 13 hr to 14 hr after the onset of photophase, diapause was almost prevented. Another slight reduction of diapause incidence occurred at 11-13 hr after the onset of scotophase. Larvae responded to supplementary illumination of even 1 lux for diapause inhibition. The present results correlated well with the photoperiodic conditions of the diapause incidence observed in the field late in October.