Xenorhabdus nematophila (Enterobacteriaceae) is the symbiotic bacterium of an Egyptian strain of entomopathogenic nematode (EPN), Steinernema carpocapsae BA2 (Rhabditidae). In this study, the symbionts bacterium, Xenorhabdu snematophila BA2, was isolated from the haemolymph of the greater wax moth larvae, Galleria mellonella, infected with a native strain of S. carpocapsae (BA2). The efficiency of a fresh isolated bacterium was compared to 2-years old bacterium against the third-instar larvae of G. mellonella L. and cotton leaf worm Spodoptera littoralis (Boisd.) under laboratory conditions. The fresh bacteria caused an average of 51.5 and 54% mortalities to 3^rd instar larvae of G. mellonella and S. littoralis, respectively. The 2-year old bacteria caused a respective of 48 and 52% mortalities. The mortality percentage increased as the concentration of entomopathogenic bacteria (EPB) increased. The LC₅₀ values for X. nematophila reached 1.64x10⁷ and 1.74x10⁷ bacterial cells/ml for fresh and stored S. littoralis, respectively and 1.71x10⁷ and 1.88x10⁷ bacterial cells/ml for fresh and stored G. mellonella, respectively.