A continuous culture system has been used to study the effects of low (sub-MIC) levels of sodium fluoride on the stability and metabolism of a defined oral microbial community. The microflora was also subjected to glucose pulses at pH 7.0, with and without subsequent pH control. At pH 7.0, a continuous supply of 1 mmol/L NaF reduced slightly the viable counts of the oral microflora, although their proportions were relatively unaffected. At pH 7.0, during glucose pulsing, 1 mmol/L NaF prevented the rise in proportions of A. viscosus and reduced the levels of B. intermedius. Glucose pulsing without pH control and in the absence of fluoride markedly inhibited the growth of many species, and L. casei, V. dispar, and S. mutans predominated in the culture. Fluoride (1 mmol/L), either pulsed with the glucose or provided continuously, reduced both the rate of change and the degree of fall in pH, and in doing so prevented the enrichment of S. mutans in the culture. Fluoride also reduced the pH-mediated inhibition of other members of the oral community, although S. sanguis was inhibited even further. Thus, even sub-MIC levels of fluoride may have a beneficial anti-bacterial effect on dental plaque by interfering with acid production. This would reduce the pH-mediated disruption to the balance of the microflora and suppress the selection of S. mutans.