SummaryThe effect of dietary xylitol on the ability of the rat caecal flora to metabolise xylitol was investigated. Xylitol metabolism in micro‐organisms has generally been assessed in terms of pH change and acid production which are often insensitive in demonstrating low rates of substrate utilisation. Using a rapid and sensitive radioisotopic assay, in which 14CO2 production from |U‐14C| xylitol was measured, it was possible to show that the caecal microflora obtained from rats can metabolise xylitol. This activity was increased 10, 15, 30 and 40‐fold in the caecal flora taken from rats fed diets containing 2.5, 5, 10 and 20% xylitol, respectively.Using the caecal microflora of normally fed rats, the order of 14CO2 production from 14C‐labelled sugars and sugar alcohols was glucose > fructose > xylose > sorbitol ⩾ xylitol. The feeding of glucose and fructose did not alter the 14CO2 producing activities, whereas xylose feeding increased xylose metabolism, sorbitol feeding increased sorbitol and xylitol metabolism, xylitol feeding increased sorbitol, xylose and its own metaboiism and arabitol feeding increased xylose and sorbitol metabolism. Marked changes were also observed in the population of the caecal flora of xylitol‐fed rats, with increases in the number of gram‐positive bacteria, compared to rats on a normal diet. Possible mechanisms for these effects involve mutation, selection of micro‐organisms capable of metabolising sugar alcohols, and the induction of enzymes involved in sugar alcohol metabolism.