Single-stage direct bioconversion of cellulosic materials to ethanol, under almost the same fermentation conditions applied to the Saccharomyces cerevisiae-ethanol process, was attempted using intergeneric fusants of Trichoderma reesei QM 9414/S. cerevisiae NCIM 3288. Optimization of medium constituents was made using the Box-Behnken design method. The combined effect of pH and temperature on the fermentation was analyzed by the central composite design method. Under optimal conditions the fusant produced 0.17 g l-1 ethanol in 30 h. © 1995.