Xylose conversion into xylitol by Candida guilliermondi FTI 20037 was investigated in a stirred tank bioreactor at different stirring rates. Maximal xylitol production (22.2 g litre(-1)) was obtained at 30 degrees C, with an aeration rate of 0.46 vvm using a stirring rate of 300 min(-1) (K(L)a=10.6 h(-1)). An increase in K(L)a produced an increase in the consumption of xylose in detriment to xylitol formation. The efficiency of substrate conversion to xylitol was 45% in a medium containing glucose and xylose but increased to 66% when glucose was omitted.