The ability of Candida parapsilosis to ferment xylose to xylitol in the presence of glucose and mannose was evaluated using a semidefined medium and aspenwood hemicellulose hydrolysate obtained by steam explosion acid hydrolysis. This natural growth substrate can be used with an appropriate dilution to give initial xylose, glucose, and mannose concentrations of 50, 27, and 6 g l(-1), respectively. Above these values, inhibition of growth and metabolic activities appeared to be dur to the level of by-products generated during pretreatment and hydrolysis of aspenwood. By supplementing the hemicellulose hydrolysate with yeast extract, xylitol yield was comparable to that measured in a semi-defined medium using a sugar mixture but the rate of xylitol production was twice as low. To improve the xylitol yield and productivity, an approach based on sugar assimilation is proposed.