||The structure of a water-insoluble polysaccharide produced by the glucosyltransferase of Streptococcus mutans 6715 has been elucidated through the use of periodate oxidation, Smith degradation, dextranase digestion, concanavalin A binding studies, methylation followed by methanolysis, reductive cleavage and gas chromatographic-mass spectroscopic analysis, carbon-13 nuclear magnetic resonance and fast atom bombardment mass spectroscopy. These studies show that the water-insoluble glucan is comprised of 67% (alpha)-(1-3) linkages in a contiguous backbone with the remaining 33% existing as (alpha)-(1-6) linkages possibly as linear residues extending from (alpha)-(1-6) branch points. 14% of the residues exist as branch points and the ratio of linear extending (alpha)-(1-3) residues in the backbone to linear extending (alpha)-(1-6) residues in the side chain was found to be 5:2. Dextranase digestion and Smith degradation both gave rise to a high molecular weight fraction which is only (alpha)-(1-3) linked. In addition, the average length of the side chains was shown to not exceed 3 residues.