The synthesis of an adherent, dextran-like, extracellular polysaccharide has been demonstrated in cell-free extracts, resting and growing cells of a strain of Lacto. casei (32-1+) isolated from the human gingival crevice. Confirmation of the dextran nature of the material was obtained by controlled periodate oxidation and borohydride reduction, chromatographic analysis and serologic criteria. Glucose units were the sole components of the polysaccharide and were joined by alpha -1,6 or alpha -1,2 linkages. The synthesizing enzyme, dextransucrase, was constitutive and was produced in growing cells from glucose as well as sucrose, although sucrose is the preferred reaction (“donor”) substrate. This dextran is distinct from the capsular heteropolysaccharide produced by some Lacto. casei strains but is identical to the dextrans of several cariogenic streptococci when examined by techniques of immunodiffusion. Chemical and physical confirmation of these serologic data were essential, however, since bands of identity were observed between these dextrans and several non-dextran polysaccharides produced by diverse oral bacteria.