Pyrethroid resistance in insects is associated with elevated P450 monooxygenase levels to detoxify pesticides. The objective of this study was to examine how resistant tarnished plant bugs, Lygus lineolaris (Palisot de Beauvois), tolerate insecticide toxicity through gene regulation. A cDNA, cloned from both pyrethroid-susceptible and -resistant strains of L. lineolaris, contained a 1548-nucleotide open reading frame encoding a 516 amino acid residue protein. Predicted cytochrome P450s from cDNAs were highly similar to several insect CYP6 P450 monooxygenases which are responsible for reduced sensitivity to pyrethroid insecticides. A total of 48 nucleotide substitutions were revealed between cDNAs of susceptible and resistant strains, 13 of which were observed on the cytochrome P450 protein coding region resulting in 12 silent substitutions and only one amino acid modification from Ser⁴⁸⁷ in the susceptible strain to Ala⁴⁸⁷ in the resistant strain. The resistant strain contained 2.1-fold higher P450 monooxygenase mRNA than the susceptible strain. Topical treatment with 10 ng permethrin elevated P450 monooxygenase mRNA levels by ~2-fold. Treatment with 10 μg piperonyl butoxide suppressed P450 monooxygenase mRNA levels by ~34%.