The combined effects of initial xylose concentration and inoculum level on xylitol production by Candida guilliermondii from rice straw hydrolysate were studied using response surface methodology. A 2(2) full-factorial central composite design was employed for experimental design and analysis of the results. The optimum xylose concentration and inoculum level were found to be 82 and 3 g/l, respectively. In these conditions, xylitol concentration of 52 g/l with a production rate of 0.54 g/l and a yield factor of 0.65 g/g was attained. These results are in close agreement with the model predictions.