The taxonomic position of nitrogen-fixing strains that were isolated from rhizosphere macerates of rice cultivated in the Binh Thanh region of Vietnam was determined by using polyphasic taxonomy. We determined the phylogenetic relationships of these organisms by performing DNA-rRNA hybridization experiments with a labeled rRNA probe from the type strain of , and we found that they belong to a single rRNA complex. Other members of this rRNA complex were also studied, and the N-fixing strains were found to be closely related to . In addition, all members of the rRNA complex containing were studied by performing auxanographic and DNA-DNA hybridization experiments. Phenotypically and genotypically, the N-fixing isolates constitute a single cluster together with two strains of clinical origin. These organisms constitute a new species, for which the name is proposed; the type strain of this species is TVV75 (= LMG 10929). All members of this species can fix nitrogen. On the basis of our polyphasic taxonomy results and previously published data we concluded that the genus should be restricted to the following species: (the type species), comb. nov., and comb. nov. On the basis of genotypic and phenotypic results [, and belong to two other clusters whose internal structures must be studied further.


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