Abstract: The potency of two Bacillus strains and four new isolates of transconjugants was determined against Spodoptera littoralis. Most of bacterial transconjugants revealed insignificant correlation between the average consumption of fed leaves with the time of treatment during the age of larvae if a mixture of relatively pure crystals and endospores was used, similar to that resulting from either fraction of crystals protein alone. This suggested that feeding was reduced by the B. thuringiensis products. The bacterial preparation containing crystals and endospores from transconjugant-D revealed high levels of toxicity for the negative correlation obtained between the average consumption of fed leaves with the larval age. Highly significant correlation was obtained between mortality and the larval age using bacterial preparations containing crystals alone from trans-conjugants A and D, as well as, that containing crystals and endospores from all bacterial strains and their transconjugants. In most cases, transconjugant preparations causing high levels of mortality than parental strains, because of high toxicity against S. littoralis larvae. The new product of transconjugant-B containing crystals and endospores appeared to have insignificant correlation between treatment time and the average weight of surviving larvae. It was indicated that this product increased pathogenicity and decreased both the growth and development of S. littoralis larvae. This new product from transconjugant-B decreased the average consumption of fed leaves and the average weight of surviving larvae and it would be preferable against S. littoralis in biological control. The results showed that higher levels of mortality, if a mixture of relatively pure crystals and spores was used as compared to either fraction of crystals protein alone.