The effect of oxygenation on ethanol production from xylose or glucose by Pichia stipitis in batch culture was investigated, The fermentation parameters with respect to ethanol production by P. stipitis were strongly dependent on the oxygen transfer rate (OTR). On the basis of OTR values, the relationship between the optimum specific oxygen uptake rate (qO(2)) and maximum specific ethanol production rate ((q(p))(max) was evaluated. The optimum qO(2) for ethanol production was 14.3 or 66.7 mg.(g cell)(-1).h(-1) when xylose or glucose was used as a carbon source, respectively. When a mixture of glucose and xylose was used as the carbon source in a culture of P. stipitis alone, the yield and productivity of ethanol were significantly enhanced when qO(2) was adjusted to an optimum level dependent upon the type of sugar consumed. In a co-culture of Saccharomyces cerevisiae and P. stipitis, it was impossible to control qO(2) at an optimum value for xylose fermentation by P. stipitis because oxygen was consumed by the S. cerevisiae. In a co-culture of P. stipitis and a respiratory-deficient mutant of S. cerevisiae, the optimum qO(2) for xylose fermentation by P. stipitis was successfully maintained due to the low oxygen consumption of the mutant yeast strain. Co-culture of these two strains resulted in the maximum yield and the highest productivity of ethanol from a mixture of glucose and xylose.