Abstract: The levels of cytochrome P450s and five indices of monooxygenase activities were compared between resistant- lambda-cyhalothrin and susceptible strains of beet armyworm, Spodoptera exigua (Hübner). The levels of cytochrome P450s in two stains were in the order of fifth instar larval midgut＞fatbody＞integument. Compared with the susceptible strain, cytochrome P450s in midgut, fatbody and integument were elevated by 1.78-, 1.54- and 1.37-fold and the activities of MROD, EROD, ECOD, AHH and AE in midguts were increased 1.33-, 1.73-, 1.40-, 1.51- and 1.30-fold, respectively, in fifth instar larva of resistant strain. This indicated that resistance of Spodoptera exigua to lambda-cyhalothrin was closely correlated with multi-function oxidase.