The thermotolerant yeast strain, Kluyveromyces marxianus IMB3, was found to be capable of ethanol production during growth at 45 degrees C on media containing milled paper and exogenously added commercial cellulase. At maximum achievable cellulose concentrations in shake-flask cultures, ethanol production increased to 6.6 g/l at 45 degrees C, representing an overall level of conversion of 21% of the maximum theoretical yield. Subsequent studies involving variations in added cellulase concentrations to the batch systems demonstrated that ethanol yields could be increased to 10 g/l at 45 degrees C, which represented 39% of the maximum theoretical yield. As a result of ethanol production at 45 degrees C in the systems examined, we suggest that the thermotolerant ethanol-producing yeast strain K. marxianus represents a novel candidate for use in simultaneous saccharification and conversion of the resulting substrates to ethanol.