Chlorella vulgaris was cultivated in wastewater discharged from a steelmaking plant with the aim of developing an economically feasible system to remove ammonia from wastewater and CO, from flue gas simultaneously. Since no phosphorus compounds existed in wastewater, external phosphate (15.3 - 46.0 g m(-3)) was added to the wastewater. After adaptation to 5% (v/v) CO2, the growth of C. vulgaris was significantly improved at a typical concentration of CO, in flue gas of 15% (v/v). Growth of C. vulgaris in raw wastewater was better than that in wastewater buffered with HEPES at 15% (v/v) CO,. CO, fixation and ammonia removal rates were estimated as 26.0 g CO2 m(-3) h(-1) and 0.92 g NH3 m(-3) h(-1), respectively, when the alga was cultivated in wastewater supplemented with 46.0 g PO43 m(-3) without pH control at 15% (v/v) CO2.