With D-xylose (50 g l(-1)) as sole carbon substrate, aerobic cultures of S. cerevisiae consumed significant amounts of sugar (26.4 g l(-1)), producing 4.0 g xylitol l(-1) but no ethanol. In the presence of a mixture of glucose (35 g l(-1)) and xylose (15 g l(-1)), yeasts consumed 1.6 g xylose l(-1) that was converted nearly stoichiometrically to xylitol. Anaerobic conditions lessened xylose consumption and its conversion into xylitol. Traces of ethanol (0.4 g l(-1)) were produced when xylose was the only carbon source, however. Agar-entrapped yeasts behaved as anaerobically-grown cultures but with higher specific rates of xylose consumption and xylitol production.