AbstractThe development of low‐cost, sustainable, and renewable energy sources has been a major focus since the 1970s. Fuel‐grade ethanol is one energy source that has great potential for being generated from biomass. The demonstration of the fermentation of biomass‐generated producer gas to ethanol is the major focus of this article in addition to assessing the effects of producer gas on the fermentation process. In this work, producer gas (primarily CO, CO2, CH4, H2, and N2) was generated from switchgrass via gasification. The fluidized‐bed gasifier generated gas with a composition of 56.8% N2, 14.7% CO, 16.5% CO2, 4.4% H2, and 4.2% CH4. The producer gas was utilized in a 4‐L bioreactor to generate ethanol and other products via fermentation using a novel clostridial bacterium. The effects of biomass‐generated producer gas on cell concentration, hydrogen uptake, and acid/alcohol production are shown in comparison with “clean” bottled gases of similar compositions for CO, CO2, and H2. The successful implementation of generating producer gas from biomass and then fermenting the producer gas to ethanol was demonstrated. Several key findings following the introduction of producer gas included: (1) the cells stopped growing but were still viable, (2) ethanol was primarily produced once the cells stopped growing (ethanol is nongrowth associated), (3) H2 utilization stopped, and (4) cells began growing again if “clean” bottled gases were introduced following exposure to the producer gas. © 2004 Wiley Periodicals, Inc.