An RNA-based stable-isotope probing method was used to identify anaerobic phenol-assimilating bacteria present in activated sludge and used to treat synthetic coke-oven wastewater. Activated sludge was fed with nitrate and (13)C-labeled or unlabeled phenol under anaerobic conditions. After the incubation period, RNA was extracted from the activated sludge and separated by isopycnic centrifugation. Bacterial rRNA in each density fraction was analyzed by reverse transcription-PCR-mediated terminal restriction fragment length polymorphism and cloning followed by sequencing. A microorganism affiliated with the genus Azoarcus was the first to obviously incorporate (13)C: this microorganism was thought to have utilized phenol directly. The microorganisms affiliated with the genera Microbulbifer, Pelagiobacter, Pseudomonas, and Thauera were the next to incorporate (13)C. Although these microorganisms were involved in phenol degradation, whether they assimilated (13)C-labeled phenol directly or indirectly, could not be determined. Some Azoarcus and Thauera strains have previously been reported to degrade phenol under denitrifying conditions, but no strains of the other three genera have been reported to do so.