The growth and inorganic nutrient removal capacity of the cyanobacterium Phormidium bohneri on domestic wastewater were investigated in outdoor 24-l triangular bioreactors with or without the presence of added inorganic phosphate. While the addition of monopotassium phosphate did result in lap to 56% more biomass production and had an influence on the kinetics of the removal of inorganic nutrients, it did not significantly affect the total rime taken for complete removal of these nutrients. In all cases, nitrogen (ammonium and nitrate) and phosphate were removed after 50 and 75 h of growth, respectively. Biomass pro ductivities of 23-57 mg d.m. l(-1) day, combined with rates of ammonium and phosphate removal lip to 20 mg l(-1) day(-1) indicated that P. bohneri has a good potential for. wastewater treatment. In the presence of added phosphate, after 40 h of growth, rates of phosphate removal became erratic, with peaks of phosphate appearing at the beginning of the daylight. These results were interpreted in relation to the complex dynamic equilibrium existing between different forms of phosphate. (C) 1997 Elsevier Science Ltd.