Enhanced detoxification is one of the important mechanisms for insecticide resistance. Most research in this field to date has focused on the role of cytochrome P450s. Our previous work revealed that flavin-dependent monooxygenases (FMOs) were involved in metabolic resistance of Spodoptera exigua. In the present study we investigated the molecular characteristics， expression patterns and oxidative activities of SeFMO on insecticides. Three FMO genes， which encode proteins with the typical FMO motifs， were cloned from S. exigua. The oxidative activities of eukaryotically expressed SeFMO enzymes were verified with the model substrate of FMO. Importantly， the SeFMOs had significantly higher oxidative activities on metaflumizone and lambda-cyhalothrin than on model substrates and other insecticides tested. The three SeFMOs were mainly expressed in the midgut， fat body and Malpighian tubules. The tissues responsible for xenobiotic metabolism and their expression characteristics were similar to those of P450s acting as detoxification genes. The study also revealed that the expression of SeFMOs could be induced by insecticide exposure， and that SeFMOs were over-expressed in a metaflumizone-resistant strain of S. exigua. These results suggest that SeFMOs are important insecticide detoxifying enzymes， and that over-expression of FMO genes may be one of the mechanisms for metabolic resistance in S. exigua.