Simultaneous saccharification and fermentation (SSF) has long been known to have benefits such as reduced cellulase product inhibition and reduced capital costs; however, the incompatibility between ideal cellulase temperatures and temperatures for ethanolgenic microorganisms reduces SSF's effectiveness.  In this study, a thermotolerant yeast strain called Kluyveromyces marxianus IMB4 was used in an SSF process using Kanlow switchgrass as a feedstock.  Switchgrass was pretreated using pressurized, liquid hot water at 200°C for 10 min.  After pretreatment, insoluble solids were separated by filtration from the liquid prehydrolyzate and washed with deionized water to remove soluble sugars and inhibitors.  The insoluble solids were then hydrolyzed using a commercial cellulase preparation and fermented by K. marxianus IMB4 using SSF.  SSF was done at 37, 41, or 45°C and pH 4.8 or 5.5 for 7 days.  Results were compared with a control of Sacharomyces cerevisiae D5A at 37°C and pH 4.8.  Fermentation by IMB4 at 45 and 41°C ceased after 3 and 4 days, respectively, when a pH 4.8 citrate buffer was used.  Fermentation continued for all 7 days using IMB4 at 37°C and the control.  When pH 5.5 citrate buffer was used, fermentation ceased after 4 days using IMB4 at 45°C.  IMB4 at 45°C, pH 5.5 achieved 78% theoretical ethanol yield after 4 days, which was 3 days faster than when the control achieved 78% yield.