Introduction of the Sym plasmid pRL1JI into the cured Rhizobium leguminosarum bv. trifolii strain RCR5 resulted in a strain, designated RBL5523, that was expected to nodulate plants of the pea cross-inoculation group. However, effective nodulation occurred only on Vicia sativa plants, not on V. hirsuta or Pisum sativum. After random Tn5 mutagenesis, a derivative of RBL5523 was isolated that effectively nodulated and fixed nitrogen on P. sativum and V. hirsuta. Characterization of the mutant, RBL5787, indicated the cell surface components, extracellular polysaccharides, lipopolysaccharides, and outer membrane proteins, as well as the pattern of Nod metabolites, were indistinguishable from those of the parental strain. To obtain an indication of the function of the mutated locus, the flanking regions were sequenced and used to perform searches in protein and nonredundant nucleotide data-bases. No significant similarity or homology with any known sequence was detected.