The gene encoding the firefly luciferase enzyme (luc) was introduced to Rhizobium leguminosarum biovar trifolii strains with a view to using the resulting bioluminescent strains to study the survival of genetically-engineered rhizobia in soil microcosms. The genetically-engineered micro-organisms (GEMs) behaved similarly to their parent strains with respect to growth rate in laboratory media and in their symbiotic performance with their host plants. No gene transfer could be detected in laboratory mating experiments. When inoculated onto a non-sterile soil the population of the GEM declined sharply from an initial cell density of 2 times 1077 g-1 soil to approach a stable cell density of approximately 3 times 102 g-1 after 150 d. Direct photography of bioluminescent rhizobia enabled the detection of colonies as small as 0.1 mm in diameter without the need for transferring colonies onto filter paper. When a Rhizobium strain carrying the luc marker on a plasmid was used as inoculant it was possible to visualize differences in colonization of the rhizosphere of white clover and ryegrass by contact print and colour transparency films. The photographic detection methods described here demonstrate the possibilities of using bioluminescent rhizobia for assessing their survival in soil, and for looking at rhizosphere populations which may be an important site for potential gene transfer.