Aims: To investigate the production of xylitol by the yeast Candida guilliermondii FTI 20037, in a bioreactor, from rice straw hemicellulosic hydrolysate with a high xylose concentration. Methods and Results: Batch fermentation was carried out with rice straw hemicellulosic hydrolysate containing about 85 g xylose l(-1), in a stirred-tank bioreactor at 30degreesC, under aeration of 1.3 vvm (volume of air per volume of medium per min) and different stirring rates (200, 300 and 500 rev min(-1)). The bioconversion of xylose into xylitol by the yeast depended on the stirring rate, the maximum xylitol yield (Y-P/S = 0.84 g g(-1)) being achieved at 300 rev min(-1), with no need to pretreat the hydrolysate for purification. Conclusions: To determine the most adequate oxygen transfer rate is fundamental to improving the xylose-to-xylitol bioconversion by C. guilliermondii. Significance and Impact of the Study: For the microbial production of xylitol to be economically viable, the initial concentration of xylose in the lignocellulosic hydrolysate should be as high as possible, as with high substrate concentrations it is possible to increase the final product concentration. Nevertheless, there are few reports on the use of high xylose concentrations. Considering a process in bioreactor, from rice straw hemicellulosic hydrolysate, this is an innovator work.