The deliberate release of genetically engineered microorganisms requires a thorough characterization of the microbes in question. For the two bioluminescent Rhizobium meliloti strains, L1 and L33 [Selbitschka et al. (1992) Mol Ecol 1:9-19; Selbitschka et al. (1995) FEMS Microbiol Ecol 16:223-232], designated for field release, the sites of genetic modifications in the chromosomes were sequenced from amplified genomic DNA. This indicated no unexpected alterations in the nucleotide sequence. The bioluminescent phenotype was stably inherited over more than 100 generations in liquid cultures. The presence of the luciferase gene in both strains did not have secondary effects on a variety of metabolic pathways as assessed by the Biolog GN system. A specific polymerase chain reaction amplification, based on the chromosomal insertion site of the luc cassette, allowed the discrimination between the two strains and thus simplifies monitoring. The RecA-deficient strain L1 showed a strongly (more than 90%) reduced ability to nodulate alfalfa in competition with its parent strain R. meliloti 2011 and its RecA(+) counterpart L33.