Symbiotic bacteria with highly effective insecticidal activities were isolated and compared with their physiological characteristics from seven species of entomopathogenic nematodes belong to Steinernamatidae and Heterorhabditidae sp., and three of them were identified as Xenorhabdus nematophilus. Culture characteristics, insecticidal activities, pretense activities and fatty acid contents of various symbiotic bactierial isolates were also examined. In the case of cell growth and insecticidal activity, XR-PC and XR-MK were superior to other species when cultured in vitro. The insecticidal activity were highest at the early exponential growth phase, and gradually decreased with time. The protease activity of XR-DR was remarkable compared to other species. In the case of HE-HY, however the pretense activity increased in parallel with cell growth. Interestingly, the fatty acid patterns of Xenorhabdus nematophilus isolated from different emtomopathogenic nematode, showed remarkable differences in their contents of 12:0, 14:0, 16:1 cia 5 and 17:0 cyclo and hydroxy and branch factty acids were varied from 2% to 15% among total fatty acid contents.