Technical Abstract: The entomopathogenic nematode-bacteria complexes Heterorhabditis bacteriophora/Photorhabdus luminescens and Steinernema carpocapsae/Xenorhabdus nematophila are mass produced for use as biological insecticides. Two geographically distinct strains of each bacterial species were isolated from their nematode partners and serially subcultured on in vitro media to assess trait stability. Subculturing resulted in a phase shift to secondary forms in one P. luminescens strain and both X. nematophila strains within ten culture cycles. When phase shift was controlled changes in the P. luminescens strains were noted after 25 cycles in liquid culture including reductions in virulence, cell and inclusion body size, inclusion body prevalence, and culture pigments. Bacterial growth rate, however, was enhanced. Conversely, the X. nematophila strains were resistant to change, with no significant alteration of any trait when phase shift was controlled. In vitro producers of the P. luminescens/H. bacteriophora must consider the consequences of bacterial trait stability when designing their mass production processes.