Polysaccharide(s) have been isolated from supernatants of broth cultures of Actinomyces viscosus , strain T6. The organism produces significant quantities (2 mg/mg of cell [dry weight], 108 mg/liter) of this polysaccharide in the absence of sucrose. Chemical analysis indicated that N -acetylglucosamine (62%) was the major component; galactose (7%), glucose (4%), uronic acid (3%), and smaller amounts of glycerol, rhamnose, arabinose, and xylose were also present. The polysaccharide(s) is only slightly soluble in aqueous solutions. Although further purification of the polysaccharides will be necessary before definitive chemical and structural studies, the formation of extracellular polysaccharides is significant because it offers a possible explanation for plaque formation by these organisms.