We have been studying the factors that control larval diapause of Sesamia nonagrioides (Levebvre), a multivolitine pest of corn, Zea mays L., in the Mediterranean. The effect of various combinations of photoperiod and temperature on the induction of larval diapause of S. nonagrioides was examined. Short daylengths (8–12 h) were the main factor inducing diapause in this species, whereas high temperatures could avert the effect of the photoperiodic signal. Daily temperature cycles with different amplitudes (differences between the thermophase [high phase] and cryophase [low phase] temperatures), could induce diapause under a photoperiod of 12:12 (L:D) h when the cryophase coincided with the scotophase (dark period). In contrast, short daylengths had no effect on diapause when the thermophase coincided with the scotophase. Moreover, greater differences between the high and the low temperatures of the thermoperiod led to a higher incidence of diapause. Diapause also occurred under 24-h thermoperiods at constant darkness; the incidence of diapause correlated with the duration of the cryophase. The thermoperiodic response curve of diapause under constant darkness was analogous to that of the photoperiodic response of a type I, or long-day insect. The cryophase of the thermoperiod must be <17.5°C to obtain a high incidence of diapause. Consequently, it appeared that a thermoperiodic threshold during the cryophase was demonstrated. Hence, both the duration and the temperature of the cryophase of the thermoperiod were important factors controlling the diapause response. The ecological implications of this behavior are discussed.