The influence of pH on the xylose reductase (XR) activity was studied. The fed-batch fermentation using exponential feeding rate was employed to produce xylitol. The feeding started when the xylose concentration in the fermenter reached 50 g/l and the pH was adjusted to 2.5, 4.0 or 6.0. The best results for XR activity (0.567 U/mg(protein)) and xylitol volumetric productivity (1.06 g/l.h) were achieved with pH 6.0.