The phylogenetic position of Rhizobium huakuii bv. renge, a symbiont of Astragalus sinicus cv. Japan (renge-sou), was studied. The following phylogenetic approaches were used: restriction fragment length polymorphism (RFLP)-polymerase chain reaction (PCR) analysis of a full-length 16S rDNA fragment, 16S rDNA analysis of the first 300-bp sequence, bacteriophage typing, and amplification of the genomic region by random primer. All the data suggest that strains of R. huakuii by. renge should be classified into subspecies of the new genus Mesorhizobium (Jarvis et al., Inter. J. System. Bacteriol., 47, 895-898, 1997) and renamed M. huakuii subsp. rengei. All the strains fell into a tight cluster which included M. loti and M. huakuii. The strains isolated from root nodules on A. sinicus were divided into three groups as follows: group I, M. huakuii subsp. rengei B3, M. huakuii subsp. rengei My6, M. huakuii subsp. rengei My7, M. huakuii subsp. rengei My3, and M. huakuii subsp. rengei OUT30020; group II, M. huakuii subsp. huakuii CCBAU103(T), M. huakuii subsp. huakuii ACCC13005, M. huakuii subsp. huakuii 7653R, and Mesorhizobium sp. N-1; group In, Mesorhizobium sp. OUT30019. All the strains isolated in Japan except strains N-1 and OUT30019 were classified into group I. Strains in group I were sensitive to bacteriophage H1 which was isolated from rice-paddy soil in Japan. Strains in groups II and III except for M. huakuii subsp. huakuii 7653R were resistant to phage H1. Rhizobium sp. ACMP18, a native symbiont of Astragalus cicer that forms nodules on A. sinicus, showed close similarity to M. huakuii subsp. huakuii CCBAU103T, and should thus be classified as a Mesorhizobium sp. Taken together, the results of the analyses indicate that M. huakuii subsp. rengei forms a subgroup which is distinct from M. huakuii subsp. huakuii strains isolated in China and that strain B3 is the type strain.