Based on the ability of bacterial associates of entomopathogenic nematodes to produce antibiotic compounds on artificial media, it has been commonly accepted that Xenorhabdus sp. and Photorhabdus sp. inhibit a wide range of invading microorganisms in insects infected with Steinernema spp. or Heterorhabditis spp. Therefore, the question of whether antibiotic compounds produced by the primary form of bacterial symbionts associated mutualistically with S. carpocapsae and H. bacteriophora explain why insect carcasses do not putrefy but provide nutritional requirements for insect parasitic rhabditoid nematodes to complete their life-cycle was examined. Laboratory bioassays of anti-bacterial activity on nutrient agar and during parasitism in larvae of Galleria mellonella have confirmed earlier observations that in virto colonies of the primary form of X. nematophilus and P. luminescens produced agar-diffusible antibiotic compounds of a broad spectrum of anti-bacterial activity; their role in parasitism seems doubtful, however. This hypothesis is supported by a low antibiotic potency of a limited spectrum of anti-bacterial activity throughout the life-cycle of the parasites, principally in Galleria infected with S. carpocapsae. Since the lack of putrefaction cannot be explained simply by antibiotic inhibition of contaminating bacterial microflora, other competition mechanisms must be operating in parasitized insects. I postulated that a rapid and massive colonization of the insect body by nematophilic bacteria creates unfavorable conditions for the growth and multiplication of bacterial (proteolytic) contaminators making the insect carcass decay-resistant. In the case of H. bacteriophora, low antibiotic activity at an early stage of parasitism could support the colonization by P. luminescens of the host.