We developed a new design photobioreactor incorporating Chlorella sp. for removal of CO2 in stack gas. Photosynthetic conversion of CO2 into Chlorella biomass was investigated in a photobioreactor, which we termed a cone-shaped helical tubular photobioreactor. The laboratory scale photobioreactor (0.48 m high X 0.57 m top diameter) was set up with a 0.255 m(2) installation area. The photostage was made from transparent polyvinyl chloride (PVC) tubing (1.6 cm internal diameter with 2 cm wall thickness and 27 m in length). The inner surface of the cone-shaped photostage (0.50 m(2)) was illuminated with a metal halide lamp, the energy input into the photostage[photosynthetically active radiation (PAR, 400-700 nm)] was 2127 KJ day(-1) (12 h day / 12 h night). The maximum daily photosynthetic efficiency was 5.67% (PAR) under an air-lift operation at a flow rate of 0.3 litre min(-1) 10% CO2 enriched air. Maximum increase of Chlorella biomass was 21.5 g dry biomass m(-2) (installation area) day(-1) or 0.68 g dry biomass litre medium(-1) day(-1). Also, a helical tubular photobioreactor for outdoor culture was constructed with a 1.1 m(2) installation area (1.2 m top diameter) and photosynthetic productivity was investigated in July, 1996.