Rhizobium huakuii bv. renge B3, a native symbiont of Astragalus sinicus, outcompeted Rhizobium sp. strain ACMP18, which was isolated from Astragalus cicer nodules, in the formation of root nodules on A. sinicus when plants were co-inoculated with these strains. The strains occupying the nodules were identified by antibiotic resistance and phage sensitivity markers and also by polymerase chain reaction (PCR) genomic fingerprintings, which were performed by using enterobacterial repetitive intergenic consensus sequences. In PCR genomic fingerprintings, the total genomic DNA isolated from pure bacterial culture and from squashed root nodules showed identical profiles, indicating that this technique can be a useful tool for identification of rhizobia in ecological studies. When Rhizobium sp. strain ACMP18 outnumbered R. huakuii by. renge strain B3 by a factor of ten, and even when strain ACMP18 was added to plants 1 week before bacterization with strain B3, the strain B3 occupied most nodules. Dually infected nodules were not observed, although Rhizobium sp. ACMP18 formed active nodules on A. sinicus when the bacterial strain was inoculated alone.