Most cultures belonging to the "Erwinia chrysanthemi group" of soft-rot bacteria form traces of a blue, extracellular, water-insoluble pigment. This pigment is generally not found in cultures of the other members of the genus Erwinia. The blue substance has been isolated and purified from three members of the E. chrysanthemi group; it has been identified as indigoidine, 5,5'-diamino-4,4'-dihydroxy-3,3'-diazadiphenoquinone-(2,2').