All species of streptococci are divided into several groups according to the ecological niches they occupy. Members of the lactic acid group are found only in the plant environment and the dairy. Members of the pyogenic group and the buccal and intestinal residents of the subthermophilic group are found in intimate association only with homeothermic hosts. Members of the virulent pyogenic group, such asStreptococcus pyogenes, S. equi, andS. infrequens, are restricted to single hosts-the human, the horse, and the pig-whereas others, such asS. agalactiae can be found in several hosts. The intestinal dwellers of the subthermophilic group,S. bovis andS. equinus, occur in several animal species and are not isolated from other sources. The superficially homogeneousS. faecalis of the "enteric" group may actually consist of a group specifically restricted to the human host, and of a second group distributed among wild animals, insects, and plants. The latter group differs from the human type by the ability to hydrolyze starch and to produce a rennin-proteolytic type of curd in milk.Streptococcus faecium is widely distributed in both the homeothermic and poikilothermic environments, but no properties which differentiate the sources are known.Streptococcus faecium biot.casseliflavus has been isolated only from insects and plants.Streptococcus uberis occurs on the bovine lips and skin. Unidentified streptococci from all environments vary widely in properties. Many resembleS. lactis, S. faecalis, andS. faecium.