Molecular resistance of Spodoptera littoralis to gamma radiation and/or fungal pathogenic Beauveria bassiana via malate dehydrogenase (Mdhs) and peroxidase (Pxs) isozymes by using native- polyacrylamide gel electrophoresis was the aim of this study. Under laboratory conditions, the pathogenicity of B. bassiana against 2nd instar larvae of S. littoralis was studied. Mortality percentage increased with the time period of fungal incubation. The LC₅₀ value of B. bassiana was (0.4×10⁵ spore/ml) after 5 days of treatment. Analysis of gel electrophoresis revealed 4 Mdhs and 6Pxs isozyme in the 6th instar larvae of S. littoralis. Tissue extracts have been assigned according to their mobility. Gamma rays revealed disorders of the Mdhs and Pxs isozyme expression and density. Mdh₁band intensity was decreased at 100 and150 Gy, while Mdh₂ band intensity was decreased only at 100 Gy. Furthermore, Px₁, Px₂ and Px₃ isozyme intensity was decreased at 100 and 150 Gy. S. littoralis exposed to 50 Gy showed resistance to radiation effects and an increase in Pxs and Mdhs bands intensity. Through the course of infection by LC₅₀ value of B. bassiana, Mdhs and Pxs were affected and disturbed. Combined treatment of 100 Gy and B. bassiana showed synergistic effects causing marked disturbance and decrease in the isozyme pattern intensity of Mdhs and Pxs or missing Px₃. However, Mdhs and Pxs isozymes intensity and expression were greatly affected after 96 h. That reflects the depression in the resistance of S. littoralis and explains the rate of mortality after 5 days post treatment.