Microbial properties of a methanogenic granular phenol-degrading sludge were characterized using the 16S rRNA/DNA-based techniques, including polymerase chain reaction (PCR) amplification, cloning, DNA sequencing, and fluorescence in situ hybridization (FISH). The sludge was sampled from an upflow anaerobic sludge blanket reactor, which removed 98% of phenol (up to 1260 mg/l) in wastewater at 26 degrees C with 12 hours of hydraulic retention. Based on DNA analysis, the Eubacteria in the sludge was composed of 13 operational taxonomy units (OTUs). Two OTUs, one resembling Clostridium and the other remotely resembling Desulfotomaculum, were likely responsible for the conversion of phenol to benzoate, which was further degraded by five Syntrophus-resembling OTUs to acetate and H2/CO2; methanogens lastly converted acetate and H2/CO2 into methane. The role of six remaining OTUs remains unclear. Overall, the sludge was composed of 26 +/- 6% Eubacteria and 74 +/- 9% methanogens, of which 54 +/- 6% were acetotrophic Methanosaetaceae, 14 +/- 3% and 3 +/- 2% were hydrogenotrophic Methanomicrobiales and Methanobacteriaceae, respectively.