A mutant strain of Pichia stipitis, FPL-061, was obtained by selecting for growth on L-xylose in the presence of respiratory inhibitors. The specific fermentation rate of FPL-061, was higher than that of the parent, Pichia stipitis CBS 6054, because of its lower cell yield and growth rate and higher specific substrate uptake rate. With a mixture of glucose and xylose, the mutant strain FPL-061 produced 29.4 g ethanol/L with a yield of 0.42 g ethanol/g sugar consumed. By comparison, CBS 6054 produced 25.7 g ethanol/L with a yield of 0.35 g/g. The fermentation was most efficient at an aeration rate of 9.2 mmoles O-2 L-1 h(-1). At high aeration rates (22 mmoles O-2 L-1 h(-1)), the mutant cell yield was less than that of the parent. At low aeration rates, (1.1 to 2.5 O-2 L-1 h(-1)), cell yields were similar, the ethanol formation rates were low, and xylitol accumulation was observed in both the strains. Both strains respired the ethanol once sugar was exhausted. We infer from the results that the mutant, P. stipitis FPL-061, diverts a larger fraction of its metabolic energy from cell growth into ethanol production.