Although the application of biodegradative genetically engineered micro organisms (GEMs) for bioremediation is very promising, the risks of their release should be assessed before their introduction into the environment. Lux-marked Ralstonia eutropha H850Lr (formerly Alcaligenes eutrophus H850Lr) was introduced into sterile and non-sterile soil microcosms at matric potentials ?2.11, ?30, ?750, and ?1500 kPa. Viable cell concentration and luminescence activity of R. eutropha H850Lr was measured by plate counting and luminometry respectively. R. eutropha H850Lr could survive better in non-sterile soil at ?30 kPa than other matric potentials. Luminescence values were closely related to viable cell concentrations indicating the usefulness of a Bioluminescence-marker system for tracking the above bacterium in the environment. Statistical analysis showed a significant effect of matric potential on viable cell concentrations. The luminescence activities in the soil environment emphasize that these factors should be considered during application of GEMs in the natural environment.