The life-span and fecundity of females of Spodoptera littoralis (Boisd.) of strains that originated from Crete and Egypt were studied in relation to delays in mating. Virgin females and those that were not mated until eight days from emergence lived longest (means of 17 and 18 days, respectively). Earlier matings progressively reduced life-span; those mated at 0 or 1 day lived for averages of only 7·5 to 10·5 days. Preoviposition periods were extended when mating was delayed, from averages of 2 and 3 days, to averages of 9 and 10 days. Both the number of eggs laid and the percentage hatching were reduced by delays in mating beyond a critical period. In the Cretan stock this critical period was two days, but in the Egyptian stock it was less than a day. Cretan females mated early in life laid, on average, 726 eggs each and produced 541 caterpillars. When mating was delayed for four and six days, only 303 eggs were laid per female and only 93 caterpillars hatched. In the Egyptian stock females mated within one day of emergence laid 1208 eggs each and produced 948 caterpillars, compared with 499 eggs and 199 caterpillars for those not mated until six or eight days after emergence. Since a two-day delay in mating reduced the number of caterpillars to between one-fifth and one-tenth of those that resulted from early matings, it would appear that a smaller caterpillar population would result from control methods that reduced the availability of males, either by mass-trapping or blanket-spraying crops with pheromone.