laboratory tests have been conducted in Bollworms Research Department, Plant Protection Research Institute (Sharkia Branch). Field experiments have been conducted at Aga, Dakahlia governorate during 2018 cotton season to evaluate the efficiency of Lufenuron, Pyridalyl and Chlorpyrifos on larval population of H. armigera (Hüb.) as well as their effect on some biological and biochemical aspects in cotton fields. Results showed that, three insecticides had toxic effect on the newly hatched larvae of H. armigera. The highest toxic effect was recorded with Lufenuron (LC50 value was 0.95ppm) followed by Chlorpyrifos (LC50 value was 4.29 ppm) while the lowest was recorded with Pyridalyl (LC50 value was 6.02 ppm). The three pesticides caused increasing in the larval and pupal mortality and decreasing the larval and pupal duration also the larval and pupal weight, pupation, sex ratio percentages, adult longevity, oviposition periods, the number of laid eggs and hatchability percentages. Also the results showed changes in the activity of carbohydrates (amylase, trehalase, invertase), transaminase (ALT & AST) enzymes, acetylcholinestriase enzyme, total soluble protein (TSP) and total lipid (TL) on the American bollworm larvae treated with the three pesticides (Lufenuron, Pyridalyl and Chlorpyrifos). Field experiments showed that, the three insecticides caused population's reduction percentages of the H. armigera larvae in cotton fields. Chlorpyrifos caused the highest reduction reached (80.06%) while, Pyridalyl caused the lowest reduction reached (65.28%) as compared with control. Mean while population reduction rate resulted from used Lufenuron insecticide reach (70.07%).