The polytypic species of nodule bacteria Rhizobium leguminosarum (includes biovars viciae and trifolii) and Neorhizobium galegae (biovars orientalis and officinalis) differ in the nucleotide polymorphism of housekeeping genes (hkg) and of symbiotically specialized genes (sym) that control the formation of N2-fixing nodules in leguminous plants. In R. leguminosarum, p-distance values for sym genes are higher than for hkg genes in strains from the same and from different biovars. In N. galegae, differences between biovars in sym genes are higher than in hkg genes while within biovars, polymorphism in sym genes is lower than in hkg genes. Coefficients of biovar differentiation for both groups of genes are higher in N. galegae than in R. leguminosarum, possibly reflecting the spatial isolation of N. galegae biovars. In these species of rhizobia, the phylogenetic congruence of sym and hkg genes is more pronounced in N. galegae than in R. leguminosarum. This difference indicates an active transfer of sym genes in R. leguminosarum populations possibly representing an important factor of the deep diversification for symbiotic traits in this rhizobia species.