Ovicidal,larvicidal and biochemical effects of Thyme oil,Thymus vulgaris,and the carbamate insecticide,Methomyl,on the cotton leafworm, Spodoptera littoralis (boisd.) (Lepidoptera: Noctuidae)were studied.The calculated percentages of reduction in egg hatching were 32.5, and 57.1% for thyme oil tested at concentrations of 2 and 4% and 21.3, and 28.2% for Methomyl tested at concentrations of 2 and 4ppm, respectively.The calculated percentages of reduction were 100% for thyme oil and Methomyl tested at concentrations of 12% and 20ppm, respectively.Larval feeding assay showed that the calculated LC50 values for thyme oil were 0.701, 0.324 and 0.108% after 24, 48 and 72hrs, respectively.The calculated LC50 values for methomyl were 1.60, 0.918 and 0.51ppm after 24, 48 and 72hrs,respectively.Topical assay showed that the calculated LC50 values were 3.601, 3.601, and 2.089% after 24, 48, and 72hrs, respectively.Among methomyl, the calculated LC50 values were 2.12, 1.28 and 0.75ppm after 24, 48 and 72hrs, respectively. The in vitro inhibitory activity of Thyme oil and Methomyl on acetylcholinesterase (AChE) were also tested in larval head and midgut of S. littoralis in this study.Concentration of 6% of Thyme oil caused 68.6 and 68.7% inhibition of AChE in heads and midgut of the 4^th larval instar of S. littoralis, respectively. While concentration of 20 ppm of methomyl caused 81.8 and 80.4% inhibition of AChE in heads and midgut of the 4^th larval instar of S. littoralis, respectively. I₅₀ values for larval midgut AChE was about 1.3 and 1.52 folds more resistant to inhibition by thyme oil and methomyl,respectively,than those of larval head AChE.