Effects of triflumuron (Baycidal 25%) on newly fifth larval instars of Sopdoptera littoralis were studied by a topical bioassay for the doses of 0.01, 0.1, 1, 5 and 10 μl/larva. The growth inhibition percentages were calculated during the experiment until 72 hours from the treatment, and were ranged between 8.7 to 58.53 %. Larval mortality percentages ranged between 40 and 82.5 %, while it was 2.5% in the control. The results showed reduction in the pupae weight values of S.littoralis, these values were between 239.25 to 178.28 mg while it weighed 250.8 mg in the control. Inhibition of adult formation percentages were calculated from larval stage until the adult formation, were ranged between 57.5, 92.5%, while it was 5% in the control. The histological studies illustrated heavily destruction of the reproductive tract of male when triflumuron were applied. Sex ratio of male (SR) was affected and ranged between 47.3 to 100%. Also, on the other hand, the lethal dose of triflumuron (LD₅₀ =0.006 μl/Larva) on chitin formation caused inhibition in the larval growth of the cotton leafworm. The chitin formation ratio value displayed 44.96 mg/gm for the control, while it was 15.38 mg/gm with LD₅₀ =0.006 μl/Larva.