The aerobic and anaerobic incubation of plaque bacteria in media consisting of saliva supernatant containing ribitol, arabinitol, sorbitol and mannitol produced a fall in pH whereas incubations and serial transfer in saliva-xylitol media did not lead to any significant change. Addition of 100 mM xylitol did not affect pH fall during incubation with 100 mM sucrose, nor did 5–100 mM xylitol alter the rate of plaque glycolysis in 25 mM glucose solutions when measured as the release of 3H 2O from [5- 3H]-glucose at pH 5.0 and 7.5. Plaque carbohydrate catabolism, when measured as the evolution of 14CO 2 from [U- 14C]-glucose and -xylitol in 25 and 250 mM sugar solutions, respectively, at pH 7.5, showed that carbon dioxide release from xylitol was 7–27 per cent of that released from glucose. However, the anaerobic oxidation of glucose to carbon dioxide only represented 4 per cent of the total glucose utilised by glycolysis in 250 mM glucose solutions and demonstrates the minor role of oxidation in plaque sugar breakdown. Xylitol had no inhibitory effect on plaque sugar metabolism but was itself oxidised, although at a slower rate than was glucose; this metabolic inertness probably contributes to its relative non-cariogenicity.