Diapause in Microplitis mediator Haliday (Hymenoptera: Braconidae) is manifested during the pupal stage and normally occurs during the winter season because of a photoperiodic response that is highly dependent on temperature. No diapause was observed at temperatures above 20°C regardless of the photoperiod. Only a small part of the population entered diapause at 20°C under short daylengths of light:dark 8:16–12:12. In contrast, when larvae were exposed to 16, 18, and 20°C combined with a photoperiod of 10:14, the percentages of parasitoids that entered pupal diapause was 97.9, 87.8, and 26.2%, respectively. Critical photoperiods were determined to be 7.03:16.97 and 12.21:11.79 at 16°C and 6.75:17.25 and 12.03:11.97 at 18°C. The second instar of the parasitoid was the most sensitive to diapause induction for the photoperiods and temperatures tested.