The influence of dietary xylitol on the quantity and quality of faecal microflora was studied in Wistar albino rats, CD-1 mice and healthy human volunteers. In animals, the effects of xylitol adaptation and of 4-wk xylitol feeding were examined. No major changes in the numbers of total aerobic or anaerobic bacteria, aerobic streptococci, anaerobic streptococci or yeasts were observed, although there was evidence of a dose-dependent decrease in the numbers of aerobic streptococci in the faeces. However, xylitol feeding caused a clear shift in the rodent faecal microbial population from Gram-negative to Gram-positive bacteria. In human volunteers a similar shift was observed even after a single 30-g oral dose of xylitol. All animals were capable of adapting to 20% dietary xylitol and an accompanying enhancement of the ability of caecal and faecal flora to utilize xylitol was observed.