A novel extractive fermentation for butyric acid production from glucose, using immobilized cells of Clostridium tyrobutyricum in a fibrous bed bioreactor, was developed by using 10% (v/v) Alamine 336 in oleyl alcohol as the extractant contained in a hollow-fiber membrane extractor for selective removal of butyric acid from the fermentation broth. The extractant was simultaneously regenerated by stripping with NaOH in a second membrane extractor. The fermentation pH was self-regulated by a balance between acid production and removal by extraction, and was kept at similar topH 5.5 throughout the study. Compared with conventional fermentation, extractive fermentation resulted in a much higher product concentration (>300 g/L) and product purity (91%). It also resulted in higher reactor productivity (7.37 g/L . h) and butyric acid yield (0.45 g/g). Without on-line extraction to remove the acid products, at the optimal pH of 6.0, the final butyric acid concentration was only similar to43.4 g/L, butyric acid yield was 0.423 g/g, and reactor productivity was 6.77 g/L . h. These values were much lower at pH 5.5: 20.4 g/L, 0.38 g/g, and 5.11 g/L . h, respectively. The improved performance for extractive fermentation can be attributed to the reduced product inhibition by selective removal of butyric acid from the fermentation broth. The solvent was found to be toxic to free cells in suspension, but not harmful to cells immobilized in the fibrous bed. The process was stable and provided consistent long-term performance for the entire 2-week period of study. (C) 2003 Wiley Periodicals, Inc.