Abstract Bacillus thuringiensis (Bt) δ-endotoxins are safe biological insecticidal proteins whose usefulness has long been recognized. The first commercialized Bt insecticidal formulations were composed of spore-crystal preparations derived from wild-type strains. These products generally have a limited insecticidal host range and several genetically modified strains have, therefore, been constructed using transformation procedures. However, addition of a new δ-endotoxin gene to strains already harboring other δ -endotoxin genes often resulted in broader-spectrum but less potent products because they produced significantly less of each of the crystal proteins. We report expression of the coding sequence of the sporulation specific cryIC gene from the non-sporulation-dependent cryIIIA promoter. Large amounts of CryIC accumulated in various Bt strains with different genetic backgrounds. Sporulation deficient SpoOA mutants, acrystalliferous derivatives and wild-type Bt strains expressing the engineered cryIII-cryIC gene were obtained. Introduction of the cryIII-cryIC gene whose product is highly active against Spodoptera littoralis into the Kto strain harboring the cryIA(c) gene active against Ostrinia nubilalis resulted in the construction of a new strain with increased potency and broader activity spectrum than the parent strain. Large amounts of each toxin were produced and the expression of the two genes seemed to be summed, presumably because the expression systems of the two genes are different. The plasmid shuttle vector used to introduce the cryIII-cryIC gene into the different Bt hosts utilizes the specific resolution site of transposon Tn 4430 to enable construction of recombinant Bt strains that are free of foreign non- Bt DNA. This should facilitate the approval and acceptance for environmental release of the insecticidal recombinant products.