Extracellular polysaccharides (EPS) synthesized by Streptococcus mutans in vitro and insoluble in the culture supernatant are shown to be mixtures of water-insoluble and water-soluble EPS. After dissolving in potassium hydroxide and subsequent neutralization only part of the EPS precipitates (insoluble fraction). Another part can be isolated from the neutralized solution by the addition of ethanol (soluble fraction). Some samples show remarkable amounts of carbohydrate material, soluble even in 70% ethanol or material which can pass dialysis membranes. The insoluble fraction shows a higher share of α-(l→3)-linkages and is less susceptible to dextranase than the soluble fraction. It is suggested that the soluble fraction might be surrounded by insoluble polysaccharide or might be linked to cell-associated glucosyl-transferase, thus becoming part of an insoluble complex. Insoluble EPS synthesized by isolated glucosyltransferase of 5. mutans showed no fractionation after alkaline treatme