Xylitol and sorbitol are effective as non-cariogenic sugar substitutes. A number of studies suggest that xylitol may have an additional, caries-reducing effect. This study examines the effect of xylitol and sorbitol, when pulsed together with glucose, on the composition and metabolism of a mixed culture of oral bacteria grown in a chemostat. In this system, xylitol had the effect of reducing the rate and extent of acid production when pulsed with glucose, compared with glucose pulsed alone, enabling acid-sensitive species to persist in greater numbers. Furthermore, the enrichment of Streptococcus mutans, observed following glucose pulses in the absence of pH control, was prevented by xylitol. In contrast, sorbitol had the opposite effects and S. mutans increased in proportion. The study provides evidence for a mechanism by which xylitol could affect the ecology of dental plaque and thereby reduce the rate of dental caries.