It has been always reported that the infective juveniles of Steinernema spp. carry their symbiotic bacteria, Xenorhabdus spp. in a special intestinal vesicle (Bird and Akhurst, 1983) whereas the infective juveniles of Heterorhabdits spp. carry their symbiotic bacteria, Photorhabdus spp. in the anterior part of the intestine (Boemare et al., 1996) and release them in the body cavity of their insect hosts.