Xylitol reduces plaque but the reduction mechanism is largely unknown. The main aim of the present study was to determine whether the xylitol‐induced reduction in the amount of plaque and the number of mutans streptococci could be demonstrated in subjects with (presumably) high levels of xylitol‐resistant (XR; not inhibited by xylitol) mutans streptococci acquired following previous xylitol consumptions. 37 healthy dental students participated in the double‐blind study. All subjects had been uncontrolled, habitual consumers of xylitol‐containing products for at least 1 yr before the study, A 1‐month washout period was followed by a 2‐week test period during which either xylitol, xylitol‐sorbitol or unsweetened chewing gum base was chewed 3–5 × a day. Plaque and saliva samples were collected at baseline and at the 2‐week point for determination of the amount of plaque, microbiological variables, and hydrolytic enzymes. Mixtures of xylitol and sorbitol seemed to perform equally well with respect to reduction in the amount of plaque but not the number of mutans streptococci. Thus, polyols were the active ingredients of chewing gums able to modulate the amount of plaque and its microbial composition. Xylitol reduced plaque with a mechanism which appeared not to be associated with the study‐induced changes in the proportion (%) of mutans streptococci in plaque, the number of salivary mutans streptococci, the proportion of XR strains in plaque or saliva, or the hydrolytic enzyme activities of plaque.