A Gram-negative, nonpigmented bacterium designated strain B1 was isolated from a laboratory bioreactor that reduced selenate to elemental red selenium (Se-0). 16S rRNA gene-sequence alignment identified the isolate as a Rhizobium sp. belonging to the Rhizobium clade, which includes R. daejeonense, R. giardinii, R. undicola, R. larrymoorei, R. radiobacter, R. rubi, and R. vitis. R. radiobacter and R. rubi are its closest relatives as indicated by 16S rRNA gene-sequence alignments, which differ from strain B1 by 2.6% and 2.8%, respectively. Within this group, strains that show variances > 0.8% to 2.2% have been classified as different species. The major cellular fatty acids present in the B1 strain were C16:0 (1.8%), C18:0 (3.38%), 18:0 3-OH (1.6%), 18:1 omega 7c (86.8%), 19:0 cyclo omega 8c (1.5%), and summed features 2 (3.8%) and 3 (1.2%). The large amount of 18:1 omega 7c present is constant with members of this group of bacteria, but the small amounts of 16:0, 19:0 cyclo omega 8c, and summed feature 3 shows variance from R. radiobacter and R. rubi. The strain's phenotypic and biochemical characteristics are consistent with its placement in this genus.