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 protein (Met), transcriptional expression of cytochrome P450 subfamilies and genes was compared between developmental stages, methoprene-sensitive (Oregon-R) and tolerant (Rst(1)JH¹) strains of Drosophila melanogaster and different doses of methoprene. In Oregon-R third instars, low levels of methoprene in the diet (3.2 ppm) was associated with decreased expression of Cyp315a1, Cyp12d1, Cyp6a2, Cyp4ac, Cyp4e2 and Cyp12a. In Rst(1)JH¹ third instars, low levels of methoprene slightly increased expression of Cyp315a1, Cyp12d1, Cyp4ac, Cyp4e2 and Cyp12a. Most striking was the complete loss of expression of Cyp6a2 in untreated and methoprene-treated Rst(1)JH¹ third instars. These preliminary analyses demonstrate that methoprene treatment can induce differential expression of P450 transcripts in D. melanogaster and that the specific responses are associated with the Met transcription factor. 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.