The effects from early and late defoliation of cotton on the numbers of overwintering pink bollworm, Pectinophora gossypiella (Saunders), were studied in the Imperial Valley of southern California. Fewer moths emerged from cotton defoliated in September than from October defoliations. Cotton defoliated in early September resulted in a 90 per cent reduction in emerging moths the following spring, while cotton defoliated in late September resulted in only a 66 per cent reduction in emerging moths.