Fluoride uptake was examined in the highly F-sensitive Streptococcus salivarius strain 25975, the F-resistant mutant Flr103, and the relatively insensitive S. sanguis H7PR3. F was taken up by all strains from media at neutral pH, containing 1 or 10 micrograms F/ml (0.053-0.526 mmol/L), and uptake was directly proportional to the delta pH that was maintained by the cells. The final intracellular concentrations of F in the F-resistant strains were higher than those in the F-sensitive strain. Similar differences in F uptake were observed in buffer at pH 5.5, although both delta pH and F uptake were higher in all strains at the lower pH. These experiments demonstrate that resistance to F does not necessarily involve the exclusion of F from the cells. The lower uptake of F by F-sensitive cells is attributed to the finding that F dissipates the transmembrane pH gradient, and this, in turn, limits the concentration of intracellular F.