This work has been carried out in order to construct a potent Bt strain active against the Egyptian cotton leaf worm S. littoralis. This work has been carried out in order to construct a potent Bt strain active against the Egyptian cotton leaf worm S. littoralis. In bioassays, cry1Ac-expressing kur-HD73 and cry1Ag-expressing Bt4 Bt strains caused mortality of S. littoralis larvae only slightly. When Cry1C and Cry1Ag were applied together, an additive effect was obtained. In the presence of only Cry1C, the LC50 was 70 ppm. In the presence of Cry1Ac, the LC50 was 240 ppm, and it was 120 ppm in presence of Cry1Ag. In presence of Cry1C co-expressed with Cry 1Ag or with Cry1Ac, the LC50 decreased to 10 ppm and 12 ppm respectively. Thus a combination of the Cry proteins of 1C and 1Ag /or 1Ac could result in effective insect control. With this approach, a combination of Cry proteins can be designed rather than discovered. Selecting host backgrounds suitable for large-scale fermentation can augment crystal protein production.