The fermentation of mixtures Of D-glucose and D-xylose by three non-traditional yeasts: Candida shehatae (ATCC 34887), Pachysolen tannophilus (ATCC 32691) and Pichia stipitis (ATCC 58376) have been studied to determine the optimal strain and initial culture conditions for the efficient production of ethanol. The comparison was made on the basis of maximum specific growth rate (mu(m)), biomass productivity, the specific rates of total substrate consumption (q(s)) and ethanol production (q(E)) and the overall yields of ethanol and xylitol. All the experiments were performed in stirred-tank batch reactors at a temperature of 30degreesC. The initial pH of the culture medium was 4.5. The highest values of mu(m). (above 0.5 h(-1)) were obtained with P stipitis in cultures containing high concentrations Of D-xylose. All three yeasts consumed the two monosaccharides in sequence, beginning with D-glucose. The values of q(s) diminished during the course of each experiment with all of the yeasts. The highest values of the specific rates of total substrate consumption and ethanol production were obtained with C shehatae (for t = 10 h, q(s) and q(E) were above 5 gg(-1) h(-1) and 2 gg(-1) h(-1), respectively), although the highest overall I ethanol yields were fairly similar with all three yeasts, at around 0.4gg(-1).