Glucose repressed xylose utilization in Candida tropicalis pre-grown on xylose until glucose reached approximately 0-5 g l(-1). In fermentations consisting of xylose (93 g l(-1)) and glucose (47 g l(-1)), xylitol was produced with a yield of 0.65 g g(-1) and a specific rate of 0.09 g g(-1) h(-1), and high concentrations of ethanol were also produced (25 g l(-1)). If the initial glucose was decreased to 8 g l(-1), the xylitol yield (0.79 g g(-1)) and specific rate (0.24 g g(-1) h(-1)) increased with little ethanol formation (<5 g l(-1)). To minimize glucose repression, batch fermentations were performed using an aerobic, glucose growth phase followed by xylitol production. Xylitol was produced under O-2 limited and anaerobic conditions, but the specific production rate was higher under O-2 limited conditions (0.1-0.4 vs. 0.03 g g(-1) h(-1)). On-line analysis of the respiratory quotient defined the time of xylose reductase induction.