The ability of various sugars to induce xylose reductase (aldose reductase, EC 1.1.1.21) and xylitol dehydrogenase (D-xylulose reductase, EC 1.1.1.9) activities in Candida guilliermondii was studied. D-Xylose was found to be the best inducer of activities of both enzymes, followed closely by L-arabinose. Very low xylose reductase activity was induced by cellobiose, D-mannose, D-glucose, D-galactose, D-fructose or glycerol. With xylitol dehydrogenase, cellobiose and D-fructose caused partial induction of enzyme activity, while negligible activity was induced by D-mannose, D-glucose, D-galactose or glycerol. Several sugars were tested for the ability to repress the induction by D-xylose of xylose reductase and xylitol dehydrogenase activities in C. guilliermondii. Enzyme activities induced on D-xylose served as controls. L-Arabinose, cellobiose and D-galactose did not repress enzyme induction by D-xylose, while D-mannose, D-glucose and D-fructose repressed enzyme induction to varying extents. Results from enzyme induction generally correlated with patterns of mixed sugar utilization, with some anomalies associated with the utilization of D-galactose and D-fructose in the presence of D-xylose. The results show that the utilization of D-xylose by C. guilliermondii is subject to regulation by induction and catabolite repression.