Insect cytochrome P450 monooxygenases (CYPs or P450s) play an important role in detoxifying insecticides leading to resistance in insect populations. A polyphagous pest, Spodoptera litura, has developed resistance to a wide range of insecticides. In the present study, a novel P450 gene, CYP321B1, was cloned from S. litura. The function of CYP321B1 was assessed using RNA interference (RNAi) and monitoring resistance levels for three commonly used insecticides, including chlorpyrifos, beta-cypermethrin and methomyl. The full-length complementary DNA sequence of CYP321B1 is 1814 bp long with an open reading frame of 1488 bp encoding 495 amino acid residues. Quantitative reverse-transcriptase polymerase chain reaction analyses during larval and pupal development indicated that CYP321B1 expression was highest in the midgut of fifth-instar larvae, followed by fat body and cuticle. The expression of CYP321B1 in the midgut was up-regulated by chlorpyrifos, beta-cypermethrin and methomyl with both lethal concentration at 15% (LC15) (50, 100 and 150 mu g/mL, respectively) and 50%(LC50) dosages (100, 200 and 300 mu g/mL, respectively). Addition of piperonyl butoxide (PBO) significantly increased the toxicity of chlorpyrifos, beta-cypermethrin and methomyl to S. litura, suggesting a marked synergism of the three insecticides with PBO and P450-mediated detoxification. RNAi-mediated silencing of CYP321B1 further increased mortality by 25.6% and 38.9% when the fifth-instar larvae were exposed to chlorpyrifos and beta-cypermethrin, respectively, at the LC50 dose levels. The results demonstrate that CYP321B1 might play an important role in chlorpyrifos and beta-cypermethrin detoxification in S. litura.