Clostridium tyrobutyricum produces butyrate, acetate, H-2, and CO2 as its main fermentation products from glucose and xylose. To improve butyric acid and hydrogen production, integrational mutagenesis was used to create a metabolically engineered mutant with inactivated ack gene, encoding acetate kinase (AK) associated with the acetate formation pathway. A non-replicative plasmid containing the acetate kinase gene (ack) fragment was constructed and introduced into C. tyrobutyricum by electroporation. Integration of the plasmid into the homologous region on the chromosome should inactivate the target ack gene and produce ack-deleted mutant, PAK-Em. Enzyme activity assays showed that the AK activity in PAK-Em decreased by similar to 50%; meanwhile, phosphotransacetylase (PTA) and hydrogenase activities each increased by similar to 40%. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) results showed that the expression of protein with similar to 32 kDa molecular mass was reduced significantly in the mutant. Compared to the wild type, the mutant grew more slowly at pH 6.0 and 37 C, with a lower specific growth rate of 0.14 h(-1) (vs 0.21 h-1 for the wild type), likely due to the partially impaired PTA-AK pathway. However, the mutant produced 23.5% more butyrate (0.42 vs 0.34 g/g glucose) at a higher final concentration of 41.7 g/L (vs 19.98 g/L) as a result of its higher butyrate tolerance as indicated in the growth kinetics study using various intial concentrations of butyrate in the media. The mutant also produced 50% more hydrogen (0.024 g/g) from glucose than the wild type. Immobilized- cell fermentation of PAK-Em in a fibrous-bed bioreactor (FBB) further increased the final butyric acid concentration (50.1 g/L) and the butyrate yield (0.45 g/g glucose). Furthermore, in the FBB fermentation at pH 5.0 with xylose as the substrate, only butyric acid was produced by the mutant, whereas the wild type produced large amounts of acetate (0.43 g/g xylose) and lactate (0.61 g/g xylose) and little butyrate (0.05 g/g xylose), indicating a dramatic metabolic pathway shift caused by the ack deletion in the mutant.