Technical Abstract: Entomopathogenic nematodes of the family Steinernematidae and their mutualistic bacteria (Xenorhabdus cabanillasii) are lethal endoparasites of insects. We hypothesized that growth of the nematode¿s mutualistic bacteria in the insect host may contribute to the production of cues used by the infective juveniles (IJ) in responding to potential hosts for infection. Specifically, we tested if patterns of bacterial growth could explain differences in CO2 production over the course of host infection. Growth of X. cabanillasii isolated from Steinernema riobrave exhibited the characteristic exponential and stationary growth phases. Other non-nematode symbiotic bacteria were also in infected hosts and exhibited similar growth patterns to Xenorhabdus. Galleria mellonella larvae infected with S. riobrave produced two distinct peaks of CO2 occurring at 25.6-36 h and 105-161 h post-infection, whereas larvae injected with Xenorhabdus alone showed only one peak of CO2, occurring at 22.8-36.2 h post-injection. Tenebrio molitor larvae infected with S. riobrave or injected with bacteria alone exhibited only one peak of CO2 production, which occurred later during S. riobrave infection (41.4-64.4 h post-infection compared to 20.4-35.9 h post-injection). These results indicate a relationship between bacterial growth and the first peak of CO2 in both host species, but not for the second peak exhibited in G. mellonella.