The effects of temperature and photoperiod on the development and diapause induction of Helicoverpa armigera (Hb. ) were examined under laboratory conditions. The larvae reared at 20°C under short as well as long photoperiods entered diapause at different rates in the pupal stage, except at 17L : 7D. The critical photoperiod for diapause induction was between 12L : 12D and 13L : 11D. Irrespective of photoperiod, the larvae reared at 15°C entered diapause, while those reared at 30°C did not. The developmental periods for eggs, larvae and pupae (non-diapausing) shortened with increasing temperature were 5.7 and 2.3, 29.0 and 10.6 days, and 25.9 and 9.2 days at 20°C and 30°C, respectively. The developmental threshold temperatures for the egg, larva and pupa were 10.8°C, 13.6°C and 14.6°C, respectively. Furthermore, the effective temperature sums for development were 45.5, 200.0 and 142.9 day-degress (D°), respectively. The heat requirements indicate that the insect may complete a maximum of four generations per year in southern Okayama Prefecture, western Japan.