Cytochrome P450 monooxygenases (P450s) are involved in the biosynthesis of endogenous intracellular compounds and the metabolism of xenobiotics, including chemical insecticides. We investigated the structural and expression level variance across all P450 genes with respect to the evolution of insecticide resistance under multigenerational dichlorodiphenyltrichloroethane (DDT) selection. RNA-sequencing (RNA-seq) and reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR) indicated that the transcript levels of seven P450 genes were significantly up-regulated and three P450 genes were down-regulated in the DDT-resistant strain 91-R, as compared to the control strain 91-C. The overexpression of Cyp6g1 was associated with the presence of an Accord and an HMS-Beagle element insertion in the 5' upstream region in conjunction with copy number variation in the 91-R strain, but not in the 91-C strain. A total of 122 (50.2%) fixed nonsynonymous (amino acid-changing) mutations were found between 91-C and 91-R, and 20 (8.2%) resulted in amino acid changes within functional domains. Three P450 proteins were truncated as a result of premature stop codons and fixed between strains. Our results demonstrate that a combination of changes in P450 protein-coding regions and transcript levels are possibly associated with DDT resistance, and thereby suggest that selection for variant function may occur within this gene family in response to chronic DDT exposure. © 2018 Society of Chemical Industry.