The photosynthetic productivity of the filamentous cyanobacterium Spirulina platensis was investigated in a cone-shaped helical tubular photobioreactor. A laboratory-scale photobioreactor was constructed with a 0.255-m(2) basal area and a conical shape (0.64 m high x 0.57 m top diameter). The photostage comprised transparent reinforced polyvinyl chloride (PVC) tubing with spirally wound, metal-wire reinforcing in the tubing wall (31 m in length and 1.6 cm internal diameter with 0.25 cm wall thickness; total volume = 6.231). The inner surface of the photostage (0.651 m(2)) was illuminated with compact fluorescent cool white lamps; the photosynthetically active radiation (400-700 nm) energy input into the photobioreactor was 1249 KJ day(-1) (12 h day/12 h night). The operation of an air-lift photobioreactor with CO2-enriched air (4%) at a flow rate of 0.31 min(-1) showed a maximum daily photosynthetic efficiency of 6.83% under batch-culture conditions. This corresponded to a production rate of 15.9 g dry biomass m(-2)(basal area) day-l or 0.51 g dry biomass 1 medium(-1) day(-1).