Many mechanisms have been proposed to explain the caries preventive effect of xylitol as a total or partial dietary sugar substitute. This article reviews the current knowledge of the effect of xylitol on the microbial population of dental plaque, particularly on mutans streptococci, in the light of an ecological concept of the oral environment and of the potential clinical significance. A noncariogenic commensal plaque flora constitutes the biotic component of a balanced ecosystem compatible with dental health. Dietary sugars, particularly sucrose, and sugar substitutes are abiotic environmental factors that can shift the delicate balance of the ecosystem towards a more or less cariogenic microbiota. Most dietary sugars are fermented by plaque microorganisms, favour the establishment of a cariogenic microflora and contribute to bacterial virulence. The vast majority of plaque bacteria, however, are incapable of fermenting xylitol into cariogenic acid end-products. There is no evidence that the plaque microbiota can adapt to metabolise xylitol or can be enriched with xylitol-metabolising cells even after long exposure to xylitol. Accumulated intracellularly as a non-metabolisable metabolite by mutans streptococci, xylitol inhibits its growth in vitro and reduces the amount of plaque and the number of mutans streptococci in both the plaque and saliva of xylitol consumers. When present in the oral environment xylitol not only prevents a shift of the bacterial community towards a more cariogenic microflora but also selects for a mutants population that was shown to have weakened virulence factors in preliminary in vitro experiments and in rats. Further research is needed to fully understand the clinical importance in the prevention of caries of this xylitol-selected population.