[Objectives]  To investigate the effect of photoperiod on the timing of eclosion in the diamondback moth (DBM), Plutella xylostella (Linn.). [Methods]  Daily eclosion activity was recorded under natural and reversed photoperiod conditions to investigate the temporal frequency of eclosion. [Results]  Adults emerged throughout the day. Eclosion had an approximately parabolic circadian rhythm; females emerged mainly during the dark phase (18:00-6:00), whereas males emerged mostly during the light phase (6:00-18:00). Reversing the photophase and scotophase disrupted the approximately parabolic eclosion rhythm resulting in no obvious pattern. No significant differences were observed in the frequency of female eclosion between the photophase and scotophase in the control group but a higher rate of male eclosion was found in the photophase than in the scotophase. After the photoperiod was reversed, more females emerged during the scotophase (06:00-18:00) than during the photophase (18:00-06:00), but fewer males emerged during the scotophase than during the photophase. [Conclusion]  Reversing the photoperiod changed the timing of eclosion of male and female DBM. These results help illustrate the relationship between photoperiod and the circadian clock in an insect pest.