The stability of biological H(2) production and microbial diversity by anaerobic sludge treated chloroform with different concentrations were investigated in this work. Taking into account the cumulative H(2) production, H(2) percentage, maximum H(2) production rate. H(2) yield and substrate utilization efficiency together, 0.050% of CHCl(3) concentration was found to be appropriate for enhancing anaerobic H(2) production. The repeated batch tests revealed a good stability of H(2) production with the inocula treated by chloroform at different concentrations. Denaturing gradient gel electrophoresis (DGGE) analysis demonstrates that CHCl(3) showed desirable selective inhibition on methanogens, rather than H(2)-producing bacteria. The favorable microbial communities for fermentative H(2) production by anaerobic sludge were uncultured bacterium clones nbt176c01, Janthinobacterium sp.7, Megasphaera paucivorans strain VTT E-03234, Clostridium cellulosi strain D3 and Clostridium sp. HPB-4. (C) 2011 Elseviei Ltd. All rights reserved.