A wide range of micro-organisms, including yeasts, was found to be inhibited by the primary form of Xenorhabdus spp., but not by the secondary form. Only one Xenorhabdus strain, the symbiont of Neoaplectana glaseri, did not inhibit any of the micro-organisms tested; it is suggested that this strain may not have been isolated in the primary form. Gram-positive bacteria were sensitive to all active isolates of Xenorhabdus; each of the yeasts and almost all of the Gram-negative bacteria were sensitive to some but not all Xenorhabdus isolates. Each Xenorhabdus isolate was sensitive to some other Xenorhabdus isolates. The antibiotic activity of X. nematophilus was unaffected by autoclaving but was lost after dialysis. Anaerobically incubated Xenorhabdus spp. did not exhibit antibiotic activity.