A fragment of CYP6B2 gene was obtained from the midgut cDNA library of the fifth instar larvae of the ballworm (Helicoverpa armigera) with RT-PCR. The product of 627 bp was cloned and sequenced. The results of dot blotting analysis of mRNA proved that CYP6B2 is expressed at higher level (mRNA) in an insecticide-resistant strain than in a susceptible strain, showing that the regulation was at transcription level. The mRNA is induced by pyrethroid in susceptible, but not in the insecticide-resistant strains. We guess that mutation in the 5' upstream regulator of the insecticide-resistant strain may lead to the difference in the expression level between resistant and susceptible strains.