The impact of bioaugmentation on the efficacy of an existing microbial population to detoxify phenol in a laboratory-scale sequencing batch reactor was evaluated. Phenol degradation and the persistence and expression of the catabolic dmpN gene were studied for 44 days using a combination of conventional monitoring approaches and molecular techniques. Following addition of the phenol-degrading bacterium, Pseudomonas putida ATCC 11172, which converts phenol to catechol by the aerobic meta-cleavage pathway, phenol removal in the bioaugmented reactor increased and was maintained at 95%-100%. In the unaugmented control reactor, decreased phenol removal was observed. Correspondingly, dmpN DNA, characteristic of P. putida ATCC 11172, and its expression were detected in activated sludge biomass from the bioaugmented reactor for over 41 days. The results of this study show that (i) bioaugmentation provides stability in phenol degradation, and (ii) monitoring with molecular techniques such as the polymerase chain reaction (PCR) and reverse transcriptase/PCR can successfully assess the state of a bacterium used in bioaugmentation.