Streptococcus strain 1A, which closely resembled Streptococcus salivarius was isolated from human dental plaque. It was found to form gelatinous plaque deposits and to induce dental caries and alveolar bone loss in gnotobiotic rats fed a sucrose-containing diet. Animals fed diet 2000 developed primarily root and interproximal smooth surface lesions, whereas animals fed died 585 developed both fissure and root lesions. Histological examination revealed that the alveolar bone was severely resorbed, possibly due in part to hypervascularization of the bone. In vitro strain 1A formed plaque-like deposits in sucrose broth, but not in broth containing glucose, fructose, glucose and fructose, or maltose. Chemical, spectrophotometric and chromatographic analysis indicated that large quantities of levan and smaller amounts of glucose-containing polysaccharides were formed from sucrose. Synthesis of dextran-like polysaccharide rather than levan appeared essential for the development of plaque-like deposits, for no deposits developed in sucrose broth supplemented with dextranase, fructose or low MW dextran. Strain 1A formed fructosyl and glucosyl transferases constitutively, and the enzymes were present in a cell-free and cell-bound form. Preparations of the cell-associated fructosyl and glucosyl transferases were active from pH 4.5 to pH 8.0, with an optimum at approximately pH 6.0. Strain 1A did not synthesize iodophilic polysaccharide for nutrient storage, but was able to degrade levan, suggesting that levan may be utilized as a nutrient reserve.