Photoperiod response of diapause induction and the sensitive stage to photoperiod was investigated in the solitary endoparasitoid Microplitis mediator (Haliday) under 16[degree]C and 18[degree]C in the laboratory. The photoperiodic response curves in M. mediator showed that this endoparasitoid is a typical long-day species with a critical day length of 12.21 h at 16[degree]C and 12.03 It at 18[degree]C. The short day length of 10 It resulted in the highest rate of diapause (98.0% at 16[degree]C and 87.7% at 18[degree]C). The percentage of diapause was 0 when light length >=14 h or =2 h. The sensitive stage to short light of 10 h at 16[degree]C was the 2nd instar (Day 5 - 8), while the 2nd instar (Day 1 - 4, Day 9 - 12, and Day 13 - 16) and the 3rd instar were the less sensitive stages, and the egg stage and the 1st instar were the insensitive stages to short light. Microplitis mediator (Braconidae). Microplitis mediator: Diapause, Diapuse induction, Photoperiod, Critical photoperiod and sensitive stage. Arthropods; Hymenopterans; Insects; Invertebrates. none