The effects of several diel and nondiel photoperiods on the induction of larval diapause of the corn stalk borer,Sesamia nonagrioides(Lefebvre), were explored. The diapause induction curve was found to be of type III (short–long day type). The photoperiodic induction of diapause was not restricted to either specific light–dark ratios or to diel photoperiods. Instead, photoperiods of =6–13 h of photophase combined with =;10–16 h of scotophase resulted in a high incidence of diapause. Light pulses of 1 and 2 h applied at different successive points in time during the 1st part of the scotophase of a diapause–inducing photoperiod of 10:14 (L:D) h greatly reduced diapause. Such pulses, applied to the last part of the scotophase, did not prevent diapause induction. Thus, 1 photosensitive time gate of the scotophase, in which illumination prevents diapause induction, exists. No instar was found to be crucially sensitive to a diapause–inducing photoperiod of short day length. The 6th instar is a partial exception in the sense that its exposure to short day length is a necessary,but not a sufficient, condition for diapause induction. In general, as the number of instars exposed to short–day photoperiod increases, the percentage diapause also increases and vice versa. The commitment (irreversible process), however, to diapause and to nondiapause development were asymmetrical phenomena, as explained in the text. A synergistic effect of short–day photoperiods and low temperatures in the induction of diapause was demonstrated.