To study the structure of microbial communities in the biological hydrogen produc- tion reactor and determine the ecological function of hydrogen producing bacteria, anaerobic sludge was obtained from the continuous stirred tank reactor (CSTR) in different periods of time, and the diversity and dynamics of microbial communities were investigated by denaturing gra- dient gel electrophoresis (DGGE). The results of DGGE demonstrated that an obvious shift of microbial population happened from the beginning of star-up to the 28th day, and the ethanol type fermentation was established. After 28 days the structure of microbial community became stable, and the climax community was formed. Comparative analysis of 16S rDNA sequences from reamplifying and sequencing the prominent bands indicated that the dominant population belonged to low G+C Gram-positive bacteria (Clostridium sp. and Ethanologenbacterium sp.), β- proteobacteria (Acidovorax sp.), γ-proteobacteria (Kluyvera sp.), Bacteroides (uncultured bacte- rium SJA-168), and Spirochaetes (uncultured eubacterium E1-K13), respectively. The hydrogen production rate increased obviously with the increase of Ethanologenbacterium sp., Clostridium sp. and uncultured Spirochaetes after 21 days, meanwhile the succession of ethanol type fer- mentation was formed. Throughout the succession the microbial diversity increased however it decreased after 21 days. Some types of Clostridium sp. Acidovorax sp., Kluyvera sp., and Bac- teroides were dominant populations during all periods of time. These special populations were essential for the construction of climax community. Hydrogen production efficiency was de- pendent on both hydrogen producing bacteria and other populations. It implied that the co- metabolism of microbial community played a great role of biohydrogen production in the reactors. To study the structure of microbial communities in the biological hydrogen producter- tion reactor and determine the ecological function of hydrogen producing bacteria, anaerobic sludge was obtained from the continuous stirred tank reactor (CSTR) in different periods of time, and diversity and dynamics of The results of DGGE demonstrated that an obvious shift of microbial population happened from the beginning of star-up to the 28th day, and the ethanol type fermentation was established. After 28 days of the structure of microbial community became stable, and the climax community was formed. Comparative analysis of 16S rDNA sequences from reamplifying and sequencing the prominent bands indicated that the dominant population belonged to low G + C Gram- positive bacteria (Clostridium sp. and Ethanologen bacterium sp.), β-proteobacteria (Acidovorax sp.), γ-proteobacteria (Kluyvera sp.), Bacteroides uncultured bacterium rims SJA-168), and Spirochaetes (uncultured eubacterium E1-K13), respectively. The hydrogen production rate increased obviously with the increase of Ethanologenbacterium sp., Clostridium sp. and uncultured Spirochaetes after 21 days, meanwhile the succession of ethanol Some types of Clostridium sp. Acidovorax sp., Kluyvera sp., and Bacteroides were formed during all periods of time. These special populations were essential for the construction of climax community. Hydrogen production efficiency was de- pendent on both hydrogen producing bacteria and other populations. It implied that the co-metabolism of microbial community played a great role of biohydrogen production in the reactors.