none Laboratory experiments were undertaken to study the relationships between the solitary endoparasitoid Microplitis rufiventris Kok. in the Egyptian cotton leafworm, Spodoptera littoralis (Boisd.) fed on the bacterial pathogen, Bacillus thuringiensis Berl. and infected with the fungus Beauveria bassiana (Bals.) Vuill. Many effects were observed on the parasitoid after treating host larvae with the entomopathogens such as: prolongation of the total developmental period, abnormality of the cocoons induced inside the host larvae and reduction in the percentage of adult emergence. Antagonism between the bacterium, B. thuringiensis and the fungus, B. bassiana in the host-parasitoid system, Spodoptera/Microplitis affected pronouncedly the mortality, as it was decreased in host and increased in the parasitoid.