Candida guilliermondii FTI 20037 was cultured in sugarcane bagasse hydrolysate supplemented with 2.0g/L of (NH4)(2)SO4, 0.1 g/L of CaCl2 . 2H(2)O, and 20.0 g/L of rice bran at 35 degreesC; pH 4.0; agitation of 300 rpm; and aeration of 0.4, 0.6, or 0.8 vvm. The high xylitol production (20.0 g/L) and xylose reductase (XR) activity (658.8 U/mg of protein) occurred at an aeration of 0.4 vvm. Under this condition, the xylitol dehydrogenase (XD) activity was low. The apparent K-M for XR and XD against substrates and cofactors were as follows: for XII, 6.4 x 10(-2)M (xylose) and 4.5 x 10(-3) mM (NADPH); for XD, 1.6 x 10(-1)M (xylitol) and 9.9 x 10(-2) mM (NAD+). Because XR requires about 10-fold less xylose and cofactor than XD for the condition in which the reaction rate is half of the V-max, some interference on the overall xylitol production by the yeast could be expected.