A polyphasic study was carried out to determine the taxonomic position of two aerobic, cyanide-degrading bacterial strains, designated L61 T and L22, which had been isolated from a bioreactor for the treatment of nickel-complexed cyanide. The two isolates exhibited almost identical taxonomic characteristics. Phylogenetic analysis inferred from comparative 16S rRNA gene sequences indicated that the isolates fall in a sublineage of the genus Rhizobium comprising the type strains of Rhizobium giardinii, Rhizobium radiobacter, Rhizobium rubi, Rhizobium larrymoorei, Rhizobium vitis, Rhizobium undicola, Rhizobium loessense, Rhizobium galegae and Rhizobium huautlense. Cells of the two isolates are Gram-negative, aerobic, motile and non-spore-forming rods (0?6-0?761?1-1?3 mm), with peritrichous flagella. The DNA G+C content is 60?1-60?9 mol%. Cellular fatty acids are C 16 : 0 (2?2-3?3 %), C 18 : 0 (2?1-3?2 %), C 19 : 0 cyclo v8c (9?9-16?8 %), C 20 : 3 v6,9,12c (2?7-3?3 %), summed feature 3 (7?2-7?7 %) and summed feature 7 (67?8-73?7 %). The strains formed nodules on a legume plant, Medicago sativa. A nifH gene encoding denitrogenase reductase, the key component of the nitrogenase enzyme complex, was detected in L61 T by PCR amplification by using a nifH-specific primer system. Strains L61 T and L22 were distinguished from the type strains of recognized Rhizobium species in the same sublineage based on low DNA-DNA hybridization values (2-4 %) and/or a 16S rRNA gene sequence similarity value of less than 96 %. Moreover, some phenotypic properties with respect to substrate utilization as a carbon or nitrogen source, antibiotic resistance and growth conditions could be used to discriminate L61 T and L22 from Rhizobium species in the same sublineage. Based on the results obtained in this study, L61 T and L22 are considered to be representatives of a novel species of Rhizobium, for which the name Rhizobium daejeonense sp. nov. is proposed. The type strain is L61 T (=KCTC 12121 T =IAM 15042 T =CCBAU 10050 T ).Since the first description of the genus Rhizobium for rootand/or stem-nodule bacteria by Frank (1889), a number of revisions and additions of novel species into this genus have been made. Currently, the genus Rhizobium includes 20 recognized species: Rhizobium galegae, R. huautlense, R. vitis, R. undicola, R. rubi, R. radiobacter, R. loessense, R. larrymoorei, R. mongolense, R. sullae, R. hainanense, R. leguminosarum, R. etli, R. tropici, R. rhizogenes, R. giardinii, R. gallicum, R. indigoferae, R. lupini and R. phaseoli.Cyanides have been regarded as toxic pollutants because of their inhibitory effect on cytochrome oxidase in respiratory electron transport chains; therefore, most countries request the complete removal of cyanide from wastewater effluent before discharge. Toxic cyanides have been known to be oxidized by various species of bacteria, fungi, yeast and plants. Isolations were made here of the cyanide-degrading bacterial population found in a bioreactor. Comparative The GenBank/EMBL/DDBJ accession numbers for...