BACKGROUND: Helicoverpa armigera (Hübner) causes severe losses in many crops including cotton. Cytochrome P450s play crucial roles in the metabolism of many important compounds in various organisms. The authors attempt to identify new cytochrome P450 genes and investigate their expression profile and xenobiotic response in order to understand the nature and roles of cytochrome P450s in this important pest.RESULTS: A novel P450 cDNA (CYP9A17), encoding a protein of 531 amino acid residues, was isolated from H. armigera. CYP9A17 is a typical microsomal P450, showing the highest (93.9%) amino acid identity with CYP9A12 from H. armigera. The high similarity is not only found between cDNAs, but also between the intron-exon organisation. CYP9A12 is constitutively universally expressed in all four life stages and in all tested larval tissues, whereas CYP9A17 is specifically expressed in the larval midgut and fat body. Real-time quantitative RT-PCR showed that the level of both CYP9A12 and CYP9A17 mRNA is affected in dose-dependent and tissue-specific manners by deltamethrin, gossypol and phenobarbital.CONCLUSION: CYP9A12 and CYP9A17 showed high sequence identity, but with differential expression patterns, suggesting that CYP9A12 and CYP9A17 genes in H. armigera might diverge via subfunctionalisation after the gene duplication event.