NolR is a reguiator of noduiation genes present in Rhizobium and Sinorhizobium. However, the mechanism by which NolR participates in the inducible transcription ofnoduiation genes remains unclear. To investigate whether there are other factors regulating the function of NoIR, an insertion mutant of NolR in Rhizobium leguminosarum strain 8401, which lacks the symbiotic plasmid, was constructed by homologous recombination. We investigated the effects of NolR inactivation on the expression of nodulation genes. Three inducible nodulation genes (nodA, nodF and nodM) were expressed constitutively in NoiR-mutant, MRl14. Our results suggested that the symbiotic plasmid is required for NolR to fully repress nodulation genes in Rhizobium ieguminosarum A34. In addition, MRl14 has provided a useful tool for further study of molecular interactions between NolR and other factors.