Butanol production by Clostridium beijerinckii NCIMB 8052 was investigated using both batch and continuous cultures containing suspended or immobilized cells. In the batch reactor, the initial addition of acetate and butyrate into the culture media was found not only to enhance solvent production but also to affect the ratio of acetone/butanol, which might result from the metabolic changes in solvent production. Furthermore, the addition of butyrate to the medium prevented strain degeneration during an extended subculturing, significantly induced butanol production (11.2 g/L butanol versus 0.45 g/L butanol with and without 36 mM butyrate, respectively), and shifted the acetone/butanol ratio to 1:3, which resulted in a higher yield (0.45 g of butanol/g of glucose) when compared to other studies. The beneficial effects of butyrate were also observed in continuous reactor tests containing suspended cells as the solvent production was maintained over more than 300 h of continuous operation. During a continuous butanol production with immobilized cells, using porous hydrophilic media and a dilution rate of 0.04 h(-1), the overall butanol productivity and yield were 0.40 g L-1 h(-1) and 0.44 g of butanol/g of glucose, respectively, which are approximately twice the values seen in a continuous reactor with suspended cells. Moreover, the butanol production was maintained over 150 days without apparent degeneration, even in the presence of high butanol concentrations (10-13 g/L). These results validate the effectiveness of producing butanol with an immobilized cell system supplemented with butyrate.