The activity of insecticide detoxifying enzyme, carboxylesterhydrolase (CEH) andmixed function oxidase (MFO)were found to be increased in P.xylostella with or without à priori exposure to B. thuringiensis subsp. kurstaki (Delfin). A significant reduction was observed in the CEH titre and MFO titre in the population of P. xylostella pre-exposed to B.thuringiensis subsp. kurstaki (Delfin) after 24 hours of post treatment with quinalophos @ 3 ppm were recorded when compared to the same without exposure to B. thuringiensis subsp. kurstaki.