Two biopesticides Serratia marcescens [Eubacteriales: Enterobacteria] (used at MC₅₀, concentration caused 50% malformation )and Bacillus thuringiensis Var. kurstaki (used at LC₅₀) and insect growth regulator Teflubenzuron (used at LC ₅₀ value) were used for treatment of 2^nd instar larvae of cotton leafworm, Spodoptera littoralis (Boisd.). Sequential combined Effect was carried out bytreating 2^nd instar larvae with LC₅₀   value of B. thuringiensis or Teflubenzuron then the larvae allowed to pupate on sawdust treated with S. marcescens at MC₅₀.The effects of these three agents were assessed by toxicity, fecundity, fertility, phosphatases activity and total carbohydrates. The obtained results revealed that Teflubenzuron is a potent toxin (LC₅₀ = 0.113 ppm) compared to B. thuringiensis (LC₅₀ = 165.64 ppm). On the other hand, the mode of action of S. marcescens is through malformation (MC₅₀= 3.09x10⁸ cfu). All treatments caused a significant reduction in fecundity and fertility.  The sequential combined effect showed more reducing effect on the fecundity and fertility than the individual treatments. Moreover, the activity of phosphatases and total carbohydrates were significantly fluctuated during the different periods of pupal stage. The sequential combined effect of Teflubenzuron with S. marcescens caused more significant effect on alkaline phosphatase activity than the individual treatment with S. marcescens but it was somewhat similar to Teflubenzuron.  Also the sequential combined effect treatment of B. thureinginsis with S. marcescens had more effect than the individual treatment either with S. marcescens or B. thureinginsis. Moreover, all treatments caused significant decreases in total carbohydrates during the pupal stage and the sequential combined effect treatments had more decreasing effect than the individual treatments. According to the obtained result, S. marcescens could be considered as a biopesticide, and become more effective when used in sequential treatment with either Teflubenzuron or B. thuringiensis.