A continuous-upflow submerged biofilter packed with ceramsite was constructed for nitrate removal under an aerobic atmosphere. Pseudomonas stutzeri X31, an aerobic denitrifier isolate, was added to the bioreactor as an inoculum. The influent NO3--N concentrations were 63.0-73.8 mg L-1. The best results were achieved when dissolved oxygen level was 4.6 mg L-1 and C/N ratio was 4.5. The maximum removal efficiencies of carbon oxygen demand (COD) and NO3--N were 94.04% and 98.48%, respectively at 30 degrees C, when the hydraulic load was 0.75 m h(-1). The top section of the bioreactor possessed less biofilm but higher COD and NO3--N removal rates than the bottom section. Polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE) technique combined with electron microscopic examination indicated P. stutzeri X31 and Paracoccus versutus were the most dominant bacteria. Amoeba sp., Vorticella sp., Philodina sp., and Stephanodiscus sp. were also found in the bioreactor. (C) 2014 Elsevier Ltd. All rights reserved.