The effects of fluoride on the initial adherence of bacteria to smooth surface enamel were evaluated in an in vivo model system. Both quantitative and qualitative changes in the colonization pattern were studied using microbiological techniques and the scanning electron microscope. Neither hourly rinsing with a 0.2% solution of NaF nor fluoride incorporated in the enamel in the form of calcium fluoride or fluorapatite influenced significantly the numbers or types of bacteria that could be recovered from the enamel test surfaces after their exposure to the oral environment for 5 h. Yet, significantly fewer bacteria, and mainly actinomycetes, were recovered from surfaces exposed hourly to a 0.373% solution of stannous fluoride. This effect may probably be ascribed to a combined adherence-interference activity of the tin ion and a bactericidal effect of the solution.