DNA-DNA hybridization in microdilution wells was successfully used to determine the overall genomic similarity among Sarothamnus scoparius microsymbionts isolated from Poland and Japan and representatives of Bradyrhizobium species including Bradyrhizobium sp. (Lupinus) USDA3045. Geographically different S. scoparius rhizobia diverged into two genomospecies at the DNA-DNA similarity level of similar to49%. Polish isolates exhibited high DNA similarity levels to the DNA of Bradyrhizobium japonicum strains (similar to78%) and formed a common genomospecies. Japanese S. scoparius rhizobia were allocated into a new genomic species due to the low similarity levels of their DNA to the DNA of representative strains of the Bradyrhizobium genus (from 19% to 52%).