Laboratory experiments were carried out to evaluate the effect of some microbial agents against the 2^nd and 4^th instar larvae of the cotton leafworm, Spodoptera littoralis (Boisd.)on tomato plants. Mortality percent of the 2^nd and 4^th instar larvae of S. littoralis increased with increasing duration after treatment with Bacillus thuringiensis kurstaki, S. littoralis NPV and Beauveria bassiana, as well as, increasing the concentration used. Positive correlation in mortality percentage of S. littoralis 2^nd and 4^th instar larvae with tested concentrations of B. thuringiensis kurstaki, whereas the higher concentration revealed higher mortality. Also, data indicated that the 2^nd instar larvae of S. littoralis was more susceptible to B. thuringiensis kurstaki, S. littoralis NPV and B. bassiana than the 4^th instar larvae