Eggs, larvae and pupae of Spodoptera littoralis (Boisd.) were exposed to five non-freezing constant temperatures (13°, 10°, 7°, 4° and 1°C). The period of exposure required to kill eggs and larvae, and to affect pupae seriously, was determined at each temperature. Distinction was made between dead pupae and pupae which were not killed but developed into deformed adults incapable of mating or laying fertile eggs. Probit line analysis was applied to the mortality results and response lines were plotted for all developmental stages at each constant temperature. Observed 100% mortality and computed LD 99 estimates showed that at all temperatures, except 1°C, resistance to cold was minimal at the egg stage, increased in maturing larvae and was at a maximum in the pupal stage, so that the order of the LD 99 values was: pupae (dead)>larvae>eggs. At 1°C the order was: pupae (dead)>30-day-old larvae>eggs>15-day-old and 1-day-old larvae. The LD 99 (days) value decreased with decreasing temperature for all larval stages and for pupae; and for eggs it was greater at 1°C than at 4° and 7°C.