The dependence of diapause induction and reactivation on thermoperiod was studied for A. manducator. Parasites were reared from the egg stage at 12.5 degree C and light-dark (LD) 12:12 or LD 18:6. The short day induced all the larvae to enter diapause, LD 18:6 induced 65-75% diapause. The diapausing insects were transferred in photoperiod LD 12:12 or LD 18:6 to one of 7 temperature regimes: constant 12.5 degree C or a thermoperiod (4 h at 27 degree C, 20 h at 12.5 degree C) with heat onset at 0, 4, 8, 12, 16, 20 h after light onset. In short day the maximum amount of developing insects was observed when high-temperature pulse was placed at the beginning and end of the night (12-16 h, 20-24 h). In long day it was observed with thermophase in the middle of the day (8-12 h). Heating at other times inhibits development. Similar effects were observed when A. manducator was raised from the egg stage in thermoperiod. After a long exposure (2-3 mo.) of the diapausing larvae to darkness and low temperature (6%) the response to thermoperiod was different. Reactivation was observed, but the high-temperature pulse in almost all regimes produced a lower proportion of developing insects than constant temperature at 12.5%. The inhibition of development induced by heating is clear-cut. none