Nitrifying bacteria,being obviously helpful to improve water quality, has been widely used in aquiculture in recent years.But the harmful substances in wastewater might seriously inhibit the activity of nitrifying bacteria,and have a strong impact on the quality of water and aquatic products.Phenol inhibition on nitrification with enriched nitrifying bacteria was studied in this paper.The results indicated that the inhibition type of phenol on nitrification was non-competitive.The inhibition constant(KI) was equal to EC50,and both were 2.61 mg/L.The effluent ammonia in the presence of phenol was higher than that of no inhibitor under the condition of the same Sludge retention time(SRT),and the higher the phenol concentration was,the higher the effluent ammonia was.The SRT of nitrifying bacteria in the presence of phenol was longer than that of no inhibitor to achieve the same effluent ammonia,and the higher the phenol concentration was,the longer the SRT was.