Objective: The aim of this study was to evaluate the in vitro antimicrobial activity of the crude aqueous extract of Piper betle by determining its effect on the growth, acid production, adherence and cell surface hydrophobicity of Streptococcus mutans. Methods: The crude aqueous extract Piper betle was prepared by decoction. Antimicrobial activity was determined by broth dilution method where the minimal inhibition concentration (MIC) was determined. To assess its effect on growth, acid production and cell adherence, the study was carried out according to the method described by Ooshima et al. (Archs Oral Biol. Vol. 45, 639-645, 2000). Cell surface hydrophobicity of S. mutans was measured as described by Saito et al. (Archs Oral Biol. Vol. 42, No. 8, 539-545, 1997). Results: The MIC value for the extract was found to be 10mg/ml. The crude aqueous Piper betle extract was found to reduce the growth rate of S. mutans, which corresponds to the reduction in its acid production. At 10 mg/ml concentration of the aqueous extract, it was found that the cell adherence property was almost completely inhibited (95.02±0.58%) (p<0.05) and the cell surface hydrophobicity was found to be reduced by 27.38±3.16 % (p<0.05). Data analysis was carried out using ANOVA. Conclusion: It can be concluded that the crude aqueous extract of Piper betle may have antiplaque property by reducing the cellular growth, acid production, cell adherence and cell surface hydrophobicity of S. mutans. This study was supported by IRPA (Project No. 09-0203-0197 EA197) and Vote F (Grant No. F0207/2003A) research grant from the Ministry of Science, Technology and Environment of Malaysia.