Sublethal doses (LD₁₀: 1.00 ng larva⁻¹; or LD₃₀: 3.75 ng larva⁻¹) of chlorfluazuron were applied topically to the cuticles of newly moulted fifth instars of the common cutworm, Spodoptera litura (F.) (Lepidoptera, Noctuidae). During mating, the treated males transferred spermatophores of a significantly lower weight into females. The weight of spermatophores transferred by LD₃₀-treated males was significantly lower than the weight of spermatophores transferred by LD₁₀-treated males, which was in turn significantly lower than the weight of spermatophores transferred by untreated males. The transfer of spermatophores was delayed by 5-15 min in LD₁₀- and LD₃₀-treated males. However, mating duration was not affected by chlorfluazuron treatment. The transfer of spermatozoa was also delayed by 5-10 min in LD₁₀- and LD₃₀-treated females. Polygynous male adults mated an average of ten times during their lifespan of 11-13 days when paired every day with a new virgin female of the same age. The number of matings per polygynous male was not affected by chlorfluazuron treatment, but the first mating was delayed by 1 day. The number of inseminated sperm found in the spermatophore averaged 10.3 (± 2.1) x 10⁵ over the lifespan of a male, in which the number of eupyrene sperm was 5.4 (± 1.1) x 10⁵. The number of inseminated eupyrene sperm decreased by 66% and 88%, respectively, in LD₁₀- and LD₃₀-treated males. No significant reduction in the number of inseminated eupyrene sperm was observed when females were treated with LD₁₀ or LD₃₀ doses, nor was there a significant reduction when both sexes were treated with the LD₁₀ or LD₃₀ doses relative to treatment of males with the same doses. The ratio of inseminated eupyrene to apyrene sperm was not affected by chlorfluazuron treatment.