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Enzymes activity included acetylcholinesterase, non-specific α and β- esterases and alkaline phosphatase were determined calorimetrically in the whole homogenates of the different instar larvae; 3rd, 4th, 5th and 6th of the field strain as well as the baseline, laboratory strain of American bollworm, Helicoverpa armigera. The results indicated that the enzymes activity levels in the different instar larvae of the field strain were markedly higher than that in the laboratory strain at the respective instar larvae. The enzymatic activity levels increased steady from the 3rd to the 6th instar larvae. A sharp increase in the enzymatic activity levels were determined in the 6th instar larvae. The average levels of the enzymatic activity in both field colony and laboratory strain were 11.630 and 3.627 µg acetyl bromide released / min. /individual for A.Ch.E.; 8.819 and 3.430 µg α-naphthol released/ min/ individual and the corresponding values were ranged between 5.232 and 3.218 µg β– naphthol released / min/ larva for beta esterase; respectively and157.669 and 50.428 unit X 103/larva for alkaline phosphatase; respectively