Fermenter experiments were carried in which the oxygen transfer coefficient (k(L)a) was varied in order to examine quantitatively the effect of oxygen on xylitol production. When the k(L)a was increased from 20 to 85 h(-1), the cell concentration increased. However, the maximum production of xylitol from 50 g/l xylose, 35.8 g/l, was achieved at 62 h at k(L)a of 30 h(-1). Cultures for xylitol production were also carried out under controlled DO concentrations. The cell concentration and xylitol dehydrogenase activity increased with increasing the DO concentration. At a high level of xylitol dehydrogenase activity, most of the xylitol was converted into xylulose, which was metabolized to cell material. The maximum xylitol production from 50 g/l xylose, 36 g/l, was obtained after 54 h and the activity of xylose reductase was maximum at a DO concentration of 0.7%, indicating that the DO concentration must be carefully controlled for efficient xylitol production from xylose. By controlling the DO concentration in the range from 0.8 to 1.2%, a final xylitol concentration of 210 g/l which corresponded to a xylitol yield of 70%, was obtained from 300 g/l xylose in 66 h.