The Photorhabdus luminescens, a symbiotic bacterium of an entomopathogenic nematode (EPNs), Heterorhabditis bacteriophora BA1 was evaluated against greater wax moth, Galleria mellonella L. and the cotton leaf worm, Spodoptera littoralis (Boise.). P. luminescens is a gram-negative bacterium, belonging to the family Enterobacteriaceae. In this study, a fresh primary phase (Phase I) of P. luminescens was isolated from a pool of 500 infective juveniles (IJs) of H. bacteriophora and its efficiency was compared to 2-years old bacterium against 3^rd larval instars of G. mellonella and S. littoralis under laboratory conditions. The virulence of P. luminescens to S. littoralis larvae was higher than that to G. mellonella larvae and mortality percentage increased as the concentration of (EPB) increased.The fresh bacteriumwasmore effective than 2-years old one. LC₅₀ were found to be 1.31x10⁸ and 1.512x10⁸ bacterial cells/ml for S. littoralis and 1.78x10⁸ and 1.95x10⁸ bacterial cells/ml for G. mellonella treated with fresh P. luminescens and 2-years old one, respectively.