A study was made of the capacity for synthesis of vitamin B12 by 70 strains representing six species of Rhizobium, namely, R. meliloti, R. trifolii, R. leguminosarum, R. japonicum, R. lupini, and R. phaseoli. Rhizobium meliloti was sharply distinguished from the other species by its ability to produce significantly higher quantities of the vitamin, one strain producing, under the experimental conditions, over 1000 mμgm. per ml. of culture liquid. The remaining species did not show significant differences in B12 synthesizing capacity. No correlation was noted between vitamin-producing capacity and effectiveness of the individual strains with respect to nitrogen fixation.