Despite the initial inhibition of growth, stepwise adaptation to fluoride was observed by strains of Streptococcus salivarius incubated in glucose–tryptone broth with NaF at concentrations as high as 14.3 mM. In medium at a variety of NaF levels up to 4.8 mM, the specific growth rates, for control cells never exposed to fluoride and for stepwise adapted strains were similar, but the rates for both cell types decreased inversely with the fluoride concentration in the medium. The lag period for the fluoride-adapted strains, however, was much shorter than the control (0-F) cells. On the other hand, when compared to stepwise adapted cells, ultraviolet-induced mutants resistant to similar levels of NaF gave higher growth rates and maximum growth values, and shorter lag periods. Furthermore, the yield of cell material per gram of glucose for the ultraviolet-induced strains was always higher, even at NaF concentrations twice that used for selection. The stoichiometry of glucose degradation to lactic acid by the ultraviolet-induced strains resembled that of the wild-type parent, while the adapted cells produced less lactic acid per mole of glucose. Despite the differences in growth and metabolic characteristics of the adapted and ultraviolet-induced mutants, both types of cells retained their resistance to fluoride even after passage for 500 generations in medium free of sodium fluoride.