Intensively cultured fish stock when fed protein-rich feeds typically excrete high concentrations of total ammoniacal-nitrogen (TAN) into the water column which can have adverse effects on productivity, and upon the environment when aquaculture water is discharged. An immobilized culture of nitrifying bacteria isolated from prawn pond water and known to effectively remove TAN from saline water was tested for its ability to remove TAN from freshwater. The culture was readily adaptable to non-saline conditions and maintained TAN at less than 0.25 mg l(-1), even with a daily addition of 3.2 to 4.2 mg TAN l(-1) per d. The use of the immobilized culture of nitrifying bacteria represents an innovative and economical in situ treatment technology for removal of TAN in both saline and freshwater.