Experiments were carried out to ascertain whether the simple agglutination test could be applied to a study of the biological specificity of Bacterium campestre, Bact. phaseoli, and Bact. phaseoli sojense. These yellow organisms, not readily distinguishable in culture, have been assumed to be closely related, but have been designated as species mainly on the basis of their host specificity, respectively, for crucifers, beans, and soybeans. In addition, Bact. flaccumfaciens, another yellow organism which may be a member of this group and which causes wilt of beans, was included in these experiments. Antiserum of each was specific for its homologous organism. In addition, antiserum of Bact. campestre gave slight agglutination in low dilutions against suspensions of the heterologous organisms; and the antisera of the other organisms, in low dilutions, gave group agglutination, when tested against suspensions of Bact. campestre. It was concluded that the simple agglutination test can be used to differentiate these 4 organisms. Though distinct serologically, Bact. campestre is closely related to Bact. phaseoli and Bact. phaseoli sojense; less closely to Bact. flaccumfaciens.