The effect of medium pH on fluoride resistance was studied using a stable, fluoride-resistant (FR) mutant of Streptococcus mutans 6715. When the mutant was grown in 750 ppm F (lethal for the wild type) and the initial pH was 7.8, significant growth occurred and the associated terminal pH was about 6.0. Growth was enhanced by graded reductions in the medium fluoride concentration and, in the absence of fluoride, growth was comparable to that of the wild type. This enhanced growth was associated with proportionate reductions in medium pH. When the initial medium pH was reduced to 6.1, growth of the wild type in fluoride-free medium was normal but that of the FR mutant in 750 ppm F was nil. Moreover, growth of the mutant at fluoride levels below 750 ppm was always reduced compared to the growth obtained when the initial pH was 7.8. With high fluoride levels, increasing the medium pH by back titration enhanced growth of the FR mutant. The results suggest that fluoride resistance is a function of the medium HF concentration.