The impact of homoacetogenesis on the fermentative hydrogen production of the anaerobic activated sludge in a Continuous Stirred-Tank Reactor (CSTR) is investigated by batch cultures. The predominant species in the sludge are identified with the technique of Poly-merase Chain Reaction-Denaturing Gradient Gel Electrophoresis(PCR-DGGE), and it is indicated that the homoacetogen Eubacterium exists in the activated sludge in a remarkable dominance. The hydrogen conversion rate of glucose and the specific hydrogen production rate by the activated sludge in terms of Mixed Liquor Volatile Suspended Solids (MLVSS) are reduced by 31% and 34%, respectively, due to the etabolism of the homoacetogens in the activated sludge. The hydrogen consumption rate of the homoacetogens in the activated sludge reaches 0.33 mmol/(g·d).