Eight Clostridium acetobutylicum strains were examined for cc-amylase and strains B-591, B-594 and P-262 had the highest activities. Defibered-sweet-potato-slurry (DSPS), containing 39.7 g starch l(-1), supplemented with potassium phosphate (1.0 gl(-1)), cysteine-HCl (5.0 gl(-1)), the antifoam (polypropylene glycol, 0.1 mg ml(-1)), was used as continuous feedstock (FS) to a multistage bioreactor system for acetone-ethanol-butanol (AEB) fermentation. The system consisted of four columns (three vertical and one near horizontal) packed with beads containing immobilized cells of C. acetobutylicum P-262. When DSPS was pumped into the bioreactor system, at a flow rate of 2.36 ml min(-1), the effluent had 7.73 g solvents l(-1) (1.56, acetone; 0.65, ethanol; 5.52 g, butanol) and no starch. Productivity of total solvents synthesized during continuous operation were 1.0 gl(-1) h(-1) and 19.5% yield compared to 0.12 gl(-1) h(-1) with 29% yield using the batch system. We propose using DSPS for AEB fermentation in a continuous mode with immobilized P-262 cells that are active amylase producers which will lead to cost reduction compared to the batch system. (C) 2001 Elsevier Science Ltd. All rights reserved.