Introduction: The crude aqueous extract of Psidium guajava L. have been shown to exhibit anti-adherence activity and reduced the adherence of plaque microorganisms to saliva-coated glass surface by 43 % (Fathilah et al, 2000). Objectives: In this study, focus was on the testing of the anti-adherence activity of the extract on selected early plaque settlers, S. sanguis, S. mitis and Actinomyces sp. These activities were first determined by the reduced population of bacteria able to bind to an extract-treated experimental pellicle on a glass surface and second by the adhering capacity of the bacteria to hexadecane. Methods: The extract-treated experimental pellicle was prepared by layering the inner surface of a borosilicate glass tube with 2ml of clarified whole saliva for 2min before it was washed with saline and exposed to 2ml of 20mg/ml Psidium guajava extract (Fathilah et al 2003). The reduction in the cell-surface hydrophobicity of the bacteria was determined by measuring the diminishing adhering capacity of the bacteria cells to hexadecane. Results: Data obtained from the study showed that the aqueous extract of Psidium guajava at 20mg/ml reduced the binding capacity of the experimental pellicle to S. sanguis, S. mitis and Actinomyces sp. by 28.1±1.8, 48.8±2.4 and 40.4±0.8 %, respectively. At 1mg/ml, the extract has also been shown to reduce the cell-surface hydrophobicity of these bacteria by 54.1±2.4, 49.9±4.8 and 40.6±2.9 %, respectively. Conclusion: With less capacity to bind to the experimental pellicle, the colonization of S. sanguis, S. mitis and Actinomyces sp. during the early phase of plaque formation may be minimized. The reduction in cell-surface hydrophobicity of the bacteria cells may also contribute to this effect. This study was supported by the Vote F Grant, University of Malaya.