Laboratory tests were carried out to determine the efficacy of two commercial preparations of Bacillus thurinigiensis, namely MVPII and Xantari, against the first instar larvae of the cotton leafworm, Spodopte­ra littoralis (Boisd.) and the black cutworm, Agrotis ipsilon(Htn.). Newly hatched, one-day and two-days old larvae of the two insects were al­lowed to feed on castor oil leaves treated with different concentrations of the two biocides for 48 hrs. Mortality among larvae of both insects increased by increasing the concentration, or post treatment period. Newly hatched larvae were the most sensitive to the toxic effect espe­cially at high concentrations and 2-days old larvae were the most toter• ant, while one-day old larvae expressed a moderate tolerance. On the other hand, S.littoralis was more sensitive than A.ipsilon, LC50 values for initial mortality were 0.0424 & 0.984 mg/ml when newly hatched larvae of the two insects were treated with MVPII and 0.0701 & 0.945 mg/ml when treated with Xantari. Two the tested formulations had also a detrimental effect of cer­tain biological aspects of the two insects. A negative relationship was observed between concentration and both pupation percentage or moth emergence. At any tested concentration, the percentages of pu­pation and moth emergence increased by increasing larval age before treatment. Also, remarkable latent adverse effects were recorded for egg-deposition as well as for egg hatchability.