From 133 saliva samples of humans we isolated 32 strains (24%) of Streptococcus sobrinus on an BYCSB selective medium. The very hard colonies of S. sobrinus on this medium were characterized by a distinct chalky white halo of water insoluble glucan ("glucan-halo") produced in large amounts from the sucrose by extracellular glucosyltransferase which has diffused out into the isolation medium. All of the isolated S. sobrinus strains fermented ordinarily mannitol and sorbitol, but not raffinose and melibiose. The occurrence of this special subspecies within the S. mutans-group may play a more important role in the etiology of dental caries of caries-active persons in the future.