The upflow aerated biofilter was used to investigate the nitrification performance of fertilizer-industry wastewater. A synthetic composition of ammonium nitrogen concentration in the range of 500-2500 mg/L was considered as a representative composition of actual fertilizer wastewater and was studied at a HRT of 24 h (corresponding applied ammonium load range was 0.5-2.5 kg NH4+-N/m(3).d). For the influent concentration increase from 500 to 2500 mg/L/NH4+-N, the ammonium removal efficiency variation was observed to be in the range of 81-58 %. The maximum ammonium removal efficiency of 81 % was observed for the applied ammonium load of 0.5 kg NH4+-N/m(3)/d The relative lower efficiencies were attributed to the probable substrate (ammonium) and/or product (nitrite) inhibition. The experiments were conducted in non-backwash conditions of the biofilter. The results were found to be satisfactory.