In order to explore the biochemical mechanisms of the beet armyworm, Spodoptera exigua (Hübner), resistance to tebufenozide, activities of detoxifying enzymes including carboxylesterase (CarE) and glutathione-S-transferases (GSTs), protective enzymes including catalase (CAT), peroxidase (POD) and superoxide (SOD), and phenoloxidase (PO) and chitinase were tested in vivo in the susceptible population, selected population and natural population in Wuhan. The results showed that CarE activities of the selected population and natural population were 1.20- and 2.67-fold, and GSTs activities were 2.3- and 0.9-fold, respectively, compared with the susceptible population. There were significant differences in Km values of the CarE but no significant differences in V_max values between the selected population and susceptible population. Compared with the selected population and susceptible population, the natural population had highly significant differences in K_m and V_max values of the CarE. PAGE results of the esterase isozymes also proved that the CarE activities and kinetic constants had changed dramatically. There were no significant differences in CAT and POD activities between the selected population and susceptible population, but SOD activity of the selected population increased obviously. Meanwhile the activities of the three enzymes in natural population also increased remarkably. The changes of SOD activities were more significant than those of CAT and POD activities. The PO and chitinase activities of the selected population increased 37.64% and 27.37%, respectively, and that of the natural population increased 59.63% and 60.29%, respectively, compared with the susceptible population. The kinetic constants of PO in the natural population and selected population had significant differences from those in the susceptible population. Refs. In Chinese