The penetration of α-, β- and γ-isomers of BHC through insect cuticle has been studied in order to find why only γ-isomer has a high insecticidal potency among the isomers. BHC-1-C14 isomers were topically applied on the male Amrican cockroaches Periplaneta americana, and radioactivities recovered from “outside” and “inside” were determined at 1.5, 6 and 24 hours after the treatment. The γ-isomer penetrates more easily than the other isomers, α- and β-isomers following in decreasing order. The rapid and easy penetration of the γ-isomer through the insect cuticle is believed to be an important factor for its insecticidal potency. It is considered, however, differeneces of certain biochemical changes among isomers which will be taken place after the penetration through insect cuticle are more importance to solve the mode of action of BHC isomers.