Fermentation characters of 60 cultures of root nodule bacteria, including members of the alfalfa, clover, pea, bean, soybean, cowpea, etc., groups, and B. radiobacter, have been determined. Agar media of low buffer capacity containing brom-thymol-blue as a pH indicator were used. Fermentation characters were sufficiently definite and pronounced to enable separation into groups corresponding to cross-inoculation groups. An exception is found in the failure to separate the soybean and cowpea groups. In many of the cross-inoculation groups, subdivisions have been established by fermentation characters, those within the alfalfa and clover groups being definite and clear cut. These groups correspond with the groups established by other workers on cultural and serological bases. Considerable variation in fermentation characters was exhibited among members of each of the other cross-inoculation groups. In these, however, differences between strains were less constant and gave less opportunity for classification into subgroups.