Production of acetone-butanol-ethanol (ABE) from potato waste starch using free and immobilized cells of Clostridium acetobutylicum was investigated. The starch was directly fermented to ABE by C. acetobutylicum without hydrolysis, and the results were compared with those obtained from glucose as a reference. Maximum butanol yields from waste starch and glucose were 0.21 and 0.26 g/g of initial carbon source (20 g/L), respectively. Batch fermentation of 60 g/L of waste starch in a 5 L bioreactor with free cells resulted in production of 9.9 g/L butanol. In a repeated batch fermentation, using immobilized cells of the bacterium in calcium alginate-polyvinyl alcohol (PVA)-boric acid beads and 60 g/L of the waste starch, a final butanol concentration of 15.3 g/L was achieved. (C) 2014 Elsevier Ltd. All rights reserved.