Phenolic compounds in fruits such as cranberries have been shown to promote a number of biological activities. The purpose of this study was to investigate the effects of polyphenolic compound-containing lingonberry extract on oral streptococci and compare them with the known anti-cariogenic activity of cranberries. Water-soluble and polyphenol-rich fractions (Fractions I and II, respectively) were isolated from cranberries and lingonberries. The effects of those fractions on the biofilm formation ability and bioactivity of Streptococcus mutans MT8148R, Streptococcus sobrinus 6715, and Streptococcus sanguinis ATCC 10556 were then evaluated. Cranberry or lingonberry Fraction II (at 0.5-1 mg/ml) significantly reduced biofilm formation by S. mutans, S. sobrinus, and S. sanguinis. In contrast, cranberry or lingonberry Fraction I (at 0.5-2 mg/ml) increased biofilm formation by S. mutans and S. sobrinus, but not by S. sanguinis. Fractions I and II (at 1-2 mg/ml) also reduced the bioactivity of S. mutans, while Fraction II (at 0.5 mg/ml) enhanced the bioactivity of all tested strains. The results revealed that lingonberries contained a larger amount of polyphenol than cranberries and that they showed almost the same level of activity against the biofilm formation ability and bioactivity of oral streptococci. This indicates that polyphenol-rich lingonberry fraction offers a promising natural food derivative for prevention of dental caries.