Aquifer flow-through columns were operated for 12 weeks to evaluate the benefits of oxygen addition on the biodegradation of a Chlroboenzene (CB) DNAPL source zone. Quantitative PCR was used to measure total bacteria (16S rDNA) and several aromatic oxygenase genes. CB removal was faster in the oxygen-amended columns compared to a control column, and qPCR showed that whereas the biphenyl and toluene dioxygenase biomarkers were most abundant, increases in the biomarker concentration for the phenol hydroxylase gene reflected best the higher CB removal due to aerobic biostimulation. DGGE analyses of the soil amended with oxygen revealed the dominant presence of Rhodococcus erythropolis (89% sequence similarity), which belongs to a genus known for its ability to degrade many priority pollutants, including CBs.