When controlled environments were programmed in bioclimatic cabinets to determine the regime that was most effective in inducing diapause in the bollworm, Heliothis zea (Boddie), and the tobacco budworm, H. virescens (F.), constant temperatures consistently were more effective than fluctuating temperatures having the same mean. A mean temperature of 21°C gave the best results. Also, a 10-hour photoperiod was optimum, but when the photoperiod exceeded 13 hours, diapause was completely inhibited in both species. Length of photoperiod had a direct effect on development of both larvae and pupae, and light intensity directly affected rate of larval development within the limits tested (25–400 foot-candles). Cotton plants as food did not increase the incidence of diapause compared with synthetic diet.