Here we report on the effects of phosphoric acid pretreated cellulose as a substrate for ethanol production by K. marxianus IMB3 using simultaneous saccharification and fermentation systems at 45 degrees C. With untreated, milled filter paper as substrate the maximum amount of ethanol produced was 25% of the maximum theoretical yield. After pre-treatment with 100% phosphoric acid, the yield increased to 42% of the maximum theoretical yield. When untreated microcrystalline cellulose was used as the fermentation substrate, yields of ethanol as 45 degrees C amounted to 16% of the maximum theoretical yield whereas pretreatment of the substrate with phosphoric acid resulted in an increase in ethanol production to 69% of the maximum theoretical yield. This suggests that pretreatment of substrate with phosphoric acid would contribute to a reduction in the amount of exogenous enzyme needed.