1. 1. The effect of nystose on cariogenic factors of mutans streptococci was investigated. 2. 2. Nystose was not utilized as a substrate for the growth, lactic acid production, plaque formation and cellular aggregation of serotype a-g strains of mutans streptococci. 3. 3. Furthermore, nystose did not served as a substrate for water-insoluble glucan formation by glucosyltransferase (GTase) from strains used. 4. 4. In the cellular adherence experiment using nystose, the amount of serotype d cells that adhered to glass surface was almost zero. 5. 5. In the presence of sucrose, nystose inhibited GTase activity and adherence of cells. 6. 6. Finally, significantly lower caries score was observed in rats infected with Strep. sobrinus and fed with diet containing nystose as compared with sucrose. © 1990.