The bollworm Helicoverpa armigera is cotton plant main pest in most parts of the world. The mechanisms of the resistance of the bollworm to the pyrethroid deltamethrin were studied by comparing field strains to the reference and susceptible strain (BK77). Resistance to deltamethrin was studied using bio-assays. Results showed that the field collected strains had susceptibility 11 to 43 fold lower than that of the susceptible standard strain BK77. Activities of two types of enzymes i.e., oxidases and Glutathione-S-Transferases (GST) were significantly higher in field strains, whereas esterase activities were lower compared to that of standard strain. The increase of oxidases and GST activities and the decrease of esterase activity are at least in part, responsible for the development of resistance of H. armigera to pyrethroids.