The gram-negative bacteriumis a Serratia marcescens ; it causes diseases in vertebrates, invertebrates and human being. Enterococcus faecalis has been isolated from Galleria mellonella and its toxicity to the same insect has been investigated. Pseudomonas plecoglossicida has been isolated from the internal organs of the diseased fish, Plecoglossus altivelis. Also, it has been reported to be causative agents of fish disease. Here, we isolate the above mentioned bacterial strains from single host (H. armigera) larvae while rearing the insect under laboratory condition. Their toxicity assays were performed and found toxic and cause insect death within 20h of treatment. Among them; S. marcescens found to be the most toxic strain against H. armigera. Also, we found that the wounded insect body are the only pave to these bacterial strains to penetrate insect body and cause the toxicity.