A Saccharomyces-cerevisiae-based simultaneous saccharification and fermentation (SSF) of lignocellulosic biomass is limited to an operating temperature of about 37 degrees C, and even a small increase in temperature can have a deleterious effect. This points to a need for a more thermotolerant yeast. To this end, S. cerevisiae D(5)A and a thermotolerant yeast, Candida acidothermophilum, were tested at 37 degrees C, 40 degrees C, and 42 degrees C using dilute-acid-pretreated poplar as substrate. At 40 degrees C, C. acidothermophilum produced 80% of the theoretical ethanol yield, which was higher than the yield from S. cerevisiae DSA at either 37 degrees C or 40 degrees C. At 42 degrees C, C. acidothermophilum showed a slight drop in performance. On the basis of preliminary estimates, SSF with C. acidothermophilum at 40 degrees C can reduce cellulase costs by about 16%. Proportionately greater savings can be realized at higher temperatures if such a high-temperature SSF is feasible. This demonstrates the advantage of using thermophilic or thermotolerant yeasts.