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The study was conducted to evaluate fermentation by <em>Clostridium thermocellum </em>and <em>C. saccharoperbutylacetonicum</em> in a continuous-flow, high-solids reactor. Liquid medium was continuously flowed through switchgrass (2 mm particle size) at one of three flow rates: 83.33&nbsp;mL&nbsp;h⁻¹ (2 L d⁻¹), 41.66 mL h⁻¹(1 L d⁻¹), and 20.833 mL h⁻¹(0.5 L d⁻¹). The cellulolytic phase was initiated by culturing <em>C. thermocellum</em> (63 °C, 24 h). The temperature was decreased (35) and <em>C. saccharoperbutylacetonicum</em> was inoculated. When metabolism decreased (96 h), the temperature was increased (63 °C; 24 h) to permit cellulosome production by <em>C. thermocellum</em>. The <em>C. saccharoperbutylacetonicum </em>was re-inoculated and the temperature returned to 35°C. The average gross production over 9 d was 1480 mg total acids (formic, acetic lactic butyric), 207 mg total solvents (acetone, butanol, ethanol), and average dry matter disappearance was 2.8 g from 25 g non-pretreated switchgrass. There was no effect of flow rate on the product formation. These results indicate that <em>C. thermocellum</em> can survive and produce cellulases with <em>C. saccharoperbutylacetonicum</em>in a continuous-flow, high-solids reactor temperature with temperature cycling.