Using a TaqMan gene probe targeting the 16S rDNA of acidophilic hydrogen-producing bacteria (HPB), the abundance of this HPB in the biomass was found to increase from 0.02 to 72% in a single batch treating rice slurry waste at pH 4.5 over 130 h. The corresponding abundances were 4.4% in the batch operated at pH 5.0 and 0.01-0.02% at pH 5.5-6.5. During the growth phase, the generation time for the acidophilic HPB at pH 4.5 averaged 3.5 h.