Thermostable cellulases are known to provide better stability and reaction rates at higher temperatures (≥60°C) which could benefit the enzymatic hydrolysis of cellulose for bioethanol production. A thermophilic cellulolytic facultative anaerobe was isolated from soil samples collected from the 4850 ft level below the surface of the Homestake gold mine in Lead, SD, now known as the NSF Deep Underground Science and Engineering Laboratory (DUSEL). The strain showed high 16S rDNA sequence homology (99%) with the genus Geobacillus and was named Geobacillus sp. R7. It produced extracellular cellulase activity when grown in a medium containing cellulose as a sole carbon source at 60°C. The growth of the R7 culture was studied at different pH and temperature in a 2L bioreactor with automated control. The crude cellulase had a pH optimum of 5 and was most active at a temperature of 80^oC. It retained about 50% and 70% of its initial activity after incubation for 7 days at 70^oC and 50^oC, respectively. The cellulase induction ability of different substrates - avicel, lactose, hardwood acid hydrolyzate, corn stover and prairie cord grass - were investigated. The last two substrates were used before and after thermo-mechanical pretreatment (extrusion at high temperature). The nutritional factors were examined for their effect on cell growth and cellulase production. Both cellulase productivity and activity were evaluated during the course of the batch fermentations. The results obtained warrant further investigations to evaluate the potential of the Geobacillus sp. R7 enzymes in the development of a simultaneous saccharification and fermentation process.