Xylitol formation by a recombinant Saccharomyces cerevisiae strain containing the XYL1 gene from Pichia stipitis CBS 6054 was investigated under three sets of conditions : (a) with glucose, ethanol, acetate, or glycerol as cosubstrates, (b) with different oxygenation levels, and (c) with different ratios of xylose to cosubstrate. With both glucose and ethanol the conversion yields were close to 1 g xylitol/g consumed xylose. Decreased aeration increased the xylitol yield on the basis of consumed cosubstrate, while the rate of xylitol formation decreased. The xylitol yield based on consumed cosubstrate also increased with increased xylose:cosubstrate ratios. The transformant utilized the cosubstrate more efficiently than did a reference strain in terms of utilization rate and growth rate, implying that the regeneration of NAD(P)(+) during xylitol formation by the transformant balanced the intracellular redox potential.