The effectiveness of an aeration-shaking (air-lift) system for outdoor biomass photoproduction by the N2-fixing filamentous blue-green alga Anabaena variabilis was evaluated and the influence of relevant factors on the productivity of the system was assessed. Air at a flow rate of 60 liters per liter of cell suspension per h was enough, by itself, to promote adequate turbulence and to supply the gaseous nutrients (CO2 and N2) needed for maximal productivity. The addition of either or both, CO2 and combined nitrogen (as KNO3 or NH4Cl), did not result in any increase in productivity. In summer and winter, optimal cell density for a suspension depth of 25 cm was 0·2–0·3 g (dry weight) liter−1 and 0·1–0·2 g (dry weight) liter−1 respectively. Reciprocally, optimal suspension depth for a cell density of 0·2 g (dry weight) liter−1 was 20–25 cm in summer and below 15 cm in winter. Optimal values for pH and temperature were 8·2–8·4 and 30–35°C, respectively. Under optimal conditions, mean productivity values were about 13 g (dry weight) m−2 day−1 in summer and 6 g (dry weight) m−2 day−1 in winter. Net protein content of A. variabilis cells was higher than 50%, and nitrogen accounted for 10% of the dry biomass. From the productivity and nitrogen content data, the N2 fixation rate in outdoor cultures of A. variabilis can be estimated to be higher than 1 g N m−2 day−1, i.e. more than 3 t N per hectare per year when values are extrapolated both in time and area.