The joint action of the pyrethroid insecticide Decis (deltamethrin)and the bioinsecticide Dipel ES (Bacillus thuringiensis) against the cabbageworm P.rapae was assessed by field plot tests. Protection of the crop and yield obtained from plots treated with several of reduced dosage mixtures were nearly similar to those recorded in plots treated with Decis or Lannate at their full rate, demonstrating a feasible proce-dure for reducing the amount of chemical insecticide applied without damaging the crop. Low concentrations of decis frequently enhanced the effect of B.thuringiensis against P.rapae on cabbage plants.