AbstractConventional acetone–butanol–ethanol (ABE) fermentation is severely limited by low solvent titer and productivities. Thus, this study aims at developing an improved Clostridium acetobutylicum strain possessing enhanced ABE production capability followed by process optimization for high ABE productivity. Random mutagenesis of C. acetobutylicum PJC4BK was performed by screening cells on fluoroacetate plates to isolate a mutant strain, BKM19, which exhibited the total solvent production capability 30.5% higher than the parent strain. The BKM19 produced 32.5 g L−1 of ABE (17.6 g L−1 butanol, 10.5 g L−1 ethanol, and 4.4 g L−1 acetone) from 85.2 g L−1 glucose in batch fermentation. A high cell density continuous ABE fermentation of the BKM19 in membrane cell‐recycle bioreactor was studied and optimized for improved solvent volumetric productivity. Different dilution rates were examined to find the optimal condition giving highest butanol and ABE productivities. The maximum butanol and ABE productivities of 9.6 and 20.0 g L−1 h−1, respectively, could be achieved at the dilution rate of 0.85 h−1. Further cell recycling experiments were carried out with controlled cell‐bleeding at two different bleeding rates. The maximum solvent productivities were obtained when the fermenter was operated at a dilution rate of 0.86 h−1 with the bleeding rate of 0.04 h−1. Under the optimal operational condition, butanol and ABE could be produced with the volumetric productivities of 10.7 and 21.1 g L−1 h−1, and the yields of 0.17 and 0.34 g g−1, respectively. The obtained butanol and ABE volumetric productivities are the highest reported productivities obtained from all known‐processes. Biotechnol. Bioeng. 2013; 110: 1646–1653. © 2013 Wiley Periodicals, Inc.