Xylitol has been claimed to reduce mutans streptococci (MS) in dental plaque by energy-consuming futile metabolic cycles. This study aimed to investigate the effects of xylitol on MS in an in vitro 6-species oral biofilm model. Each multispecies biofilm contained either a laboratory reference strain, a fresh isolate, a xylitol-sensitive or a xylitol-resistant strain of Streptococcus mutans or Streptococcus sobrinus . Biofilms, grown on pellicle-coated hydroxyapatite discs, were fed with a glucose/sucrose-supplemented medium 3 times daily for 45 min and incubated in saliva between feedings. Before or after feeding, biofilms were exposed to either 7.5% xylitol, 7.5% sorbitol or to saliva (control) for 20 min. After 64.5 h, biofilms were harvested and the microbial composition was analysed by non-selective and selective culturing. Strain variability in the ability to colonize biofilms was observed. However, the response patterns in the biofilms to the 4 polyol treatments were similar. None of the MS were inhibited by xylitol provided either before or after feeding. Sorbitol given before feeding did not affect microbial growth whereas sorbitol provided after feeding showed a slight, albeit statistically significant increase in MS counts for some of the tested strains. It did so at the expense of Streptococcus oralis, which decreased in numbers. The present findings do not support the contention that xylitol reduces MS in plaque by futile metabolic cycles.