The Efficiency of the bioinsecticide, Agerin (Bacillus thuringiensis) alone and its mixtures with three insecticides on cotton leafworm Spodoptera littoralis ( Boisd.) was investigated . Data indicated that Lufenuron was the highest effective insecticides against both the 2^nd and the 4^th instar larvae of S. littoralis followed by Chlorfluazuron , Chlorpyrifos and B.t , respectively .   All tested mixtures decreased the toxicity according to Co-Toxicity factor . The tested mixture (B.t + Chlfluazuroorn) gave the high level of antagonism effect followed by the mixture (B.t + Lufenuron) . Concerning the joint action of combination (B.t + Chlorpyrifos) , it produced additive effect when tested on the 2^nd instar larvae, while it gave antagonism effect  when tested on the 4^th instars larvae of S. littoralis . The field efficiency of tested insecticides (when used singular) can be arranged according to the general mean of reduction percentage during two seasons in a descending order as follows : Chlorfluazuron , Chlorpyrifos , Lufenuron and B.t  they were 89.26, 82.60, 77.83 and 67.15%,  respectively . The addition of B.t to Chlorpyrifos gave raising the general mean of reduction percentage for two seasons, in contrast the other two tested insecticides gave a reduction but less when applied without added to Bt . The same trend were obtained when applied three tested insecticides at the half recommended rate alone or mixed with the same dose of Bt on S. littoralis but with less level of reduction .