The paper provides a detailed bacteriologic description of the predominant flora of supragingival plaque collected from 15 caries-free Tanzanian children exposed to negligible (0.3 ppm) and excessive (3–21 ppm) concentrations of fluoride in the water supply. The three predominant groups of bacteria in all samples were streptococci, veillonellae, and pleomorphic, gram-positive rods, which together accounted for 86.8% (mean) of cultivable flora. Streptococcus mitior was the single most predominant streptococcal species. Streptococcus mutans, mainly of serotype b, was isolated from two thirds of the samples and accounted for 2.3–21.4% of the streptococcal flora. Bacteria, which potentially metabolize lactate, i. e. veillonellae, neisseriae, propionibacteria, arachniae, Bacteroides asaccharolyticus, and Streptococcus jaecalis, comprised almost one half of the total flora (range 9–76%), and may conceivably be responsible for the negligible caries activity, despite the presence of highly acidogenic bacteria. There were no detectable differences in the flora of plaque samples from children exposed to high and low water fluoride concentrations, respectively. Previously observed fluoride-induced differences in the net metabolic product, and contents of extra- and intracellular polysaccharides of plaque, is probably to be explained by a direct interference of fluoride with plaque en