Second instar larvae of Spodoptera. littoralis were fed on castor bean leaves treated with different concentrations of Dipel (Bacillus thuringiensis var. kurstaki), Helban (chemical insecticide) and combination of Dipel with LC₁₀ of Helban, 1% sodium chloride ( NaCl  ) or Dipel with 1 % sodium bicarbonate( NaHCO₃ ). Larvae fed for 24 hours on treated castor bean  leaves  dipped  for  one minute at each concentration used of the tested materials. The LC₅₀ value of Helban was 10.26 p.p.m (estimated after 24h.from treatment) ,while the LC₁₀ was 5.7 p.p.m., bioinsecticide treatment proved to be effective against 2^nd instar larvae of S. littoralis where, the recorded LC₅₀ value was15.2 × 10⁴ I.U after 5 days of treatment. While the LT₅₀ value was 4.78 and 2.75 days at concentrations of 16 and 20 × 10⁴ I.U., respectively. In case of Dipel + 1 % NaHCO₃, this treatment did not offer effect more than Dipel treatment only, where the estimated LC₅₀ value was 14.92 × 10⁴ I.U. after 5 days of treatment. While the LT₅₀ values were 4.78 and 2.03 days at concentrations of 16 and 20 × 10⁴ I.U., respectively. The most effective toxic treatment among all treatments were that of Dipel + 1% NaCl where the estimated LC₅₀ value of this treatment was 5.39 × 10⁴  I.U. 5 days post treatment , While the LT₅₀ values  were 4.831, 3.081 and 2.096 days at concentrations of 4,8 and 12 × 10⁴ I.U., respectively. To evaluate the joint action of the bioinsecticide Dipel combined with LC₁₀ of Helban two methods were followed, the first by determining the LC₅₀ values and the second by estimating the Co-toxicity factor. The LC₅₀ values were 6.5 × 10⁴ I.U. + 5.7 p.p.m. The low concentrations of Dipel when mixed with LC₁₀ of Helban produced potentiation, while the high concentrations produced additional effects.