Resistance to the juvenile hormone analog, methoprene, in Drosophila melanogaster has been identified as a target site mutation in a bHLH-PAS protein similar to the mammalian aryl hydrocarbon receptor (AhR), which regulates cytochrome P450 genes in vertebrates. To determine if cytochrome P450 genes are regulated by methoprene and/or the methoprene receptor (Met), transcriptional expression of cytochrome P450 subfamilies and genes was compared between developmental stages, methoprene-sensitive (Oregon-R, Canton-S) and tolerant (Rst(1)JH¹) strains of Drosophila melanogaster and different doses of methoprene. Cytochrome P450 subfamilies and genes were chosen for RT-PCR analysis because of known expression patterns during larval (CYP4G15, CYP310A1, CYP4AC), pupal (CYP310A1, CYP12A) or adult stages (CYP313A, CYP309A), an association with xenobiotic resistance in drosophilid insects (CYP6A, CYP6D, CYP9B, CYP28D, CYP4D) or a known function in ecdysone biosynthesis (CYP314A1, CYP306A1). In Oregon-R larvae, expression of CYP313A, CYP309A1, and CYP12A increased with increasing doses of methoprene in the diet, while CYP4D, CYP310A1, and CYP4G15 were constitutively expressed and did not change. Expression of CYP6A, CYP6D, CYP9B, CYP28D, or CYP306A1 was undetectable in third instar larvae. Using the P450 genes that respond to methoprene, we can begin to identify shared regulatory motifs that bind the methoprene receptor or other regulatory proteins in the regulatory cascade. Identifying how cytochrome P450 genes respond to methoprene will help in our understanding of the molecular mechanisms of both insecticide resistance and metamorphosis in insects.