The xylose-fermenting yeasts Pichia stipitis, Candida steatolytica, and Candida shehatae were subjected to fermentations in synthetic media containing mixtures of glucose and xylose. In all cases, repression of xylose uptake by glucose was observed, although the extent of repression was different with each yeast. While Candida shehatae was found to utilize xylose effectively in the presence of approximately 5% (w/v) glucose, Candida steatolytica could only utilize xylose when the glucose concentration was below 3% (w/v), and Pichia stipitis required the glucose concentration in the medium to be below 2% (w/v) before significant xylose utilization occurred.