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In this paper, the outdoor production of Botryococcus braunii in pilot-scale panel reactors (0.4 m(3)) is studied under uncontrolled conditions at a location close to the Atacama Desert (Chile). Discontinuous experiments were performed on different dates to determine the feasibility of the culture and the influence of environmental conditions on the system yield. Data showed that solar radiation is a major parameter in determining system yield, the average irradiance inside the culture determining both the growth rate and biomass productivity. A maximum specific growth rate of 0.09 day(-1) and biomass productivity of 0.02 g L-1 day(-1) (dry weight) were measured in discontinuous mode, at an average irradiance of 60 mu E m(-2) s(-1). With respect to lipids, a productivity of 2.5 mg L-1 day(-1) was obtained under favourable growth conditions; no accumulation of lipids at the stationary phase was observed. To confirm this behaviour, a semicontinuous culture was performed at 0.04 day(-1) in a larger reactor (1 m(3)). In this experiment, the biomass concentration and productivity was 0.3 g L-1 and 0.015 g L-1 day(-1), respectively. The lipid content and productivity was 15.6% and 2.4 mg L-1 day(-1), respectively, the mean average irradiance inside the reactor being 60 mu mol photons m(-2) s(-1). The light path of the reactor determines the light availability, thus determining also the biomass concentration and productivity of the reactor once the dilution rate is fixed. Experimentally, biomass productivity of 0.015 g L-1 day(-1) was determined for a light path of 0.15 m, but this can be increased by more than three times for a light path of 0.1 m. These data confirm that this alga can be produced outdoors in a secure form, the culture yield improving when optimal conditions are applied, the data reported here establishing the starting point for the development of the process.