NADPH-dependent aldose reductase activity induced by D-xylose or L-arabinose was detected in cell-free extracts of Candida guilliermondii, but only negligible activities were observed if D-glucose served as carbon source. The induction of aldose reductase activity on mixed sugars was investigated under resting cell conditions. D-Glucose repressed enzyme induction by D-xylose or L-arabinose to varying degrees, and L-arabinose inhibited enzyme induction by D-xylose. During incubation in a mixture of D-xylose-D-glucose, glucose consumption by cells was fast and simultaneous with D-xylose utilization. Repression of D-xylose consumption by D-glucose was dependent on hexose initial concentration. L-arabinose consumption was poor when it was present as the only sugar and in a mixture with D-glucose; this pentose depletion occurred only when all hexose was consumed. When D-xylose and L-arabinose were present in a mixture, the consumption of both pentoses was reduced by the presence of the second sugar, although both sugars were consumed simultaneously by cells. The results show that induction of aldose reductase activity and D-xylose utilization by cells of Candida guilliermondii are under control of glucose repression.