Xenorhabdus sp. (X. sp.) and Photorhabdus temperata subsp. temperata (Ptt) are the symbiotic bacteria of the entomopathogenic nematodes, Steinernema monticolum and Heterorhabditis megidis, respectively. When the nematodes enter target insects, the symbiotic bacteria are released into the insect hemocoel. The bacteria cause septicemia and kill the insect host. When ingested, the bacteria caused high mortality of third-instar larvae of Spodoptera exigua, but did not show such high pathogenicity to the fifth-instar larvae. The high mortalities in the third-instar larvae seem to be caused by antibiotic activity against Bacillus cereus, a gut symbiont required for optimal development of S. exigua. To increase their pathogenicity in the fifth instar, the bacteria should be delivered into the hemocoel. To this end, Bacillus thuringiensis aizawai (Bt) as a synergist was used to facilitate entry of the bacteria from the gut lumen into the hemocoel of S. exigua by its disruption of the insect gut epithelium. The bacterial mixture treatment was highly synergistic against the fifth-instar larvae of S. exigua. The synergistic effects were shown by the successful infection of X. sp. or Ptt in the insect hemocoel. This research shows a possibility that Xenorhabdus or Photorhabdus can be applied to kill S. exigua by oral treatment in a mixture with Bt.