Based on DNA-cloning analysis, the microbial community of a phototrophic sludge producing H-2 from an acidified wastewater was composed of 81% of a species resembling Rhodobacter capsulatus (with 99.2% similarity) and two unidentified species of the Bacillus/Clostridium group. The sludge produced a biogas comprising 82 +/- 2% H2, 13 +/- 2% CO2, 4.5 +/- 0.5% N-2, and 0.5 +/- 0.2% H2S.