Although fluoride toothpastes are widely used for caries prevention, little is known about the impact of fluoride dentifrices on plaque composition. Also the issue of adaptation of mutans streptococci to grow in vivo in a fluoride environment has received little attention. Such an adaptation may be of interest as it has been suggested that adapted mutans streptococci may show reduced glycolytic activity thereby being less cariogenic. In the present experiments the impact of the suspension of the use of fluoride toothpastes on plaque composition, fluoride tolerance and acid production of mutans streptococci was studied. Pooled plaque samples from the lingual surfaces of the lower incisors were collected from individuals (n = 13) just before and 7 weeks after they had replaced their fluoride toothpastes (0.1-0.15% F) with a non-fluoride one. The samples were analysed for fluoride and the numbers and proportions of streptococci, Streptococcus mutans, Streptococcus sobrinus, Actinomyces species, and lactobacilli, respectively. The fluoride tolerance of the mutans streptococci was estimated by culture of the plaque samples on TYCSB agar supplemented with of 0, 1, 2, 3, 4, and 5 mmol/l fluoride (NaF) at pH 7.2. From each plaque sample six S. mutans strains were isolated for the measurement of the rate of acid production (Vap) at pH 7 in the presence of 0, 5, and 10 mmol/l F. The overnight final pH was measured in cultures of the S. mutans strains with excess of glucose and 0, 5, and 10 mmol/l F. The results showed that the removal of the fluoride pressure from plaque did not affect the numbers or proportions of the various species and genera of bacteria. The fluoride tolerance of the mutans streptococci, and the Vap or the overnight final pH of the isolated strains had not changed. These results suggest that the use of fluoride toothpaste had not affected plaque composition, nor fluoride tolerance or acidogenicity of mutans streptococci. Probably the amount of fluoride delivered by fluoride dentifrices to dental plaque is too low to induce such adaptations.