Sodium fluoride (NaF) and stannous fluoride (SnF2), at concentrations of 100 ppm F-, were used as mouthrinses to determine their effect on bacterial colonization of tooth enamel in vivo. Mouthrinsing was performed either once or twice a day for up to 7 days. The enamel specimens were examined by scanning (SEM) and transmission electron microscopy (TEM). NaF when used once a day appeared to have little influence on 2-day plaque development, but when used twice a day detachment of bacteria was seen. SnF2 used either once or twice a day reduced the microbiota in early plaque formation drastically. It appeared that the variation in microbial colonization was due to alteration of adhesion of bacteria to enamel, and bacteria to bacteria.