A Gram-negative streptococcus is associated with one form of fatal septicemia arising naturally in Macacus rhesus confined in laboratories. This streptococcus differs from Streptococcus pyogenes in its behavior to Gram's method of staining; in its slight fermentative powers; in the persistence of vitality for a very long time in its cultures; in its high degree of pathogenicity for certain lower animals; and in the preservation of its virulence undiminished for many months when cultivated upon artificial media. In the present state of our knowledge this streptococcus must be considered as not identical with any streptococcus heretofore described.