The first step in the metabolism of d-xylose by yeasts and mycelial fungi was found to be the reduction of d-xylose to xylitol, a reaction catalyzed by NADPH-linked d-xylose reductase. This step is followed by the oxidation of xylitol to d-xylulose which is catalyzed by NAD-linked xylitol dehydrogenase. This oxidation-reduction reaction appears to be an obligatory step in d-xylose metabolism since direct isomerization of d-xylose to d-xylulose does not occur. The d-xylulose formed is then phosphorylated to d-xylulose-5-phosphate, the key intermediate. Similar metabolic routes have also been suggested for other aldopentoses. This enzyme system in yeasts enables them to assimilate pentoses to produce pentitols as the major metabolic products. Recently yeast mutants that exhibit different product patterns have been isolated. Studies of the enzyme activities of a xylose-utilizing yeast, Candida sp. C2, and its ethanol-producing mutant XF217 have been conducted. The specific activities of xylitol dehydrogenase and xylulokinase in XF217 increased significantly over those of the parent strain, C2. d-Xylose reductase activity remained the same. The increased xylitol dehydrogenase and xylulokinase activities enable this strain to shift from xylitol to ethanol production. Instead of excreting xylitol as a final product, more xylitol is converted to d-xylulose and ultimately to ethanol.