Microbial community structure in a lab-scale quinoline-degrading anaerobic biofilm reactor was dissected in comparison with the seeding sludge to identify the functionally important members. Samples were collected from the biofilm when the reactor reached the stable quinoline and COD removal efficiency. Total DNA was extracted from the samples. 16S rDNA v3 region was amplified and DGGE analysis was performed. The dominant bands were excised and the nucleotide sequences of the rRNA genes were determined. At the same time, a near-full length 16S rDNA library was constructed. Nucleotide sequences of 100 randomly selected clones were determined and identified by nearest neighbor analysis. Comparison analysis with the seeding sludge indicated a significant increase of the Gamma Proteobacteria and Desulfobacter postgatei during the acclimation period in the reactor, this suggested that these microorganisms may be functionally important for quinoline-degradation under anaerobic conditions.