Ethanol was produced from SO2-impregnated steam-pretreated willow by simultaneous saccharification and fermentation (SSF). Experiments with SSF were performed in stirred 300-ml vessels and in a 3-1 fermenter. Commercial cellulases were used in combination with either Saccharomyces cerevisiae or Zymomonas mobilis. The amounts of substrate, enzymes, yeast, bacteria, and nutrients were varied. The temperature was maintained at 37 degrees C, and the pH at 5.0. The results indicate that it is possible to reach over 85% of the theoretical ethanol yield, based on the glucan available in the raw material, in 3 days. This was achieved with a substrate concentration of 10% (wt/wt) and a yeast concentration of 10 g l(-1); but even with 1 and 3 g l(-1) yeast, yields of 84.7 and 80%, respectively, were reached. A cellulase concentration corresponding to 18 filter paper U g(-1) substrate was sufficient to complete SSF in 3 days. Zymomonas mobilis gave slightly reduced ethanol yields; however, a reduction in the by-product formation was also obtained compared with the yeast. The nutrient concentration could be considerably reduced compared with literature data without any decrease in the ethanol yield. An increase of substrate concentration from 10 to 15% (wt/wt) reduced the ethanol yield to below 80% as a result of insufficient mass transfer. It was shown that SSF could be performed successfully under nonsterile conditions; however, the substrate and SSF medium must be sterilized prior to SSF.