Heterorhabditid nematodes kill their hosts with the aid of a symbiotic bacterium (Photorhabdus spp.) carried within the intestine. The stability of these bacteria in culture can be critical to the nematode's virulence and their success in mass production. We investigated changes in protein electrophoresis diagrams during continuous in vitro subculturing of P. luminescens. Significant changes were observed in protein arrays during 21 culture cycles in either Phase I or Phase II bacteria isolated from H. bacteriophora (GA strain), the level of change appeared to increase as the number of culture cycles increased. In contrast, bacteria from H. bacteriophora (NJ strain) showed less change. A search on the FASTA Sequence Comparison revealed that some of changeable proteins have novel N-terminal sequence whereas others have substantial similarity with the proteins from other sources.