Newly moulted fourth instar larvae of the cotton leaf worm Spodopteralittoralis ( Boisd) were treated with Dipel 2x (Bacillus thurigiensisvarkerstaki) ( B.t.) at different concentration ( 0.5, 1.0, 1.5 and 2%) to evaluate the susceptibility of this instar and to determine the latent effects of these concentrations on some biological aspects.Its clear from our results that cumulative mortality percentage was increased gradually with an increase in the applied concentration of microbial agent.The present results indicated that the response of larvae to different concentrations of B.t. is represented by straight regression lines indicating homogeneity.The tested concentrations induced significant prolongation in the total larvae duration. There was an inverse relationship between the applied concentrations and the pupation percentage. The obtained results clearly indicated that the tested concentrations induced significant reduction in the pupal weight. And significant increase in the pupal duration as also detected.Insignificant decrease in the percentage of adult emergence was also observed and the sex ratio seams not be affected by concentration of 1.5% since it always very close to 1:1.B.t. at concentration of 2.0% caused great reduction in the fecundity of result female moths. On the other hand, the lowest reduction was observed at concentration of 0.5%. The present results revealed that the fertility of deposited eggs ( % of eggs hatch) was decreased as compared to control ( untreated larvae). The present results and observations showed that deformations of larvae, pupae and malformation of adults were occurred as a latent effect of larval treatment.