The kinetics and enzymology Of D-Xylose utilization are studied in micro-, semi-, and aerobic batch cultures during growth of Candida guilliermondii and Candida parapsilosis in the presence of several initial xylose concentrations. The abilities of xylitol accumulation by these two yeast strains are high and similar, although observed under various growth conditions. With Candida parapsilosis, optimal xylitol production yield (0.74 g/g) was obtained in microaerobiosis with 100 g/L of xylose, whereas optimal conditions to produce xylitol by Candida guilliermondii (0.69 g/g) arose from aerobiosis with 300 g/L of sugar. The different behavior of these yeasts is most probably explained by differences in the nature of the initial step of xylose metabolism: a NADPH-linked xylose reductase activity is measured with a weaker NADH-linked activity. These activities seem to be dependent on the degree of aerobiosis and on the initial xylose concentration and correlate with xylitol accumulation.