The relationship between the tilt angle of a flat-plate photobioreactor and productivity of Spirulina platensis was evaluated along with the annual seasons under the climatic conditions of south Israel (latitude approx. 31 degrees). The reactor tilt angle exerted a significant effect on the optimal population density and thus on the productivity of cell mass, owing to its control over the amount of solar radiation entering the reactor. A direct relationship between solar energy and productivity was observed : the higher the amount of solar energy that was admitted by varying the reactor tilt angle according to season, the higher was the productivity that could be sustained in the culture. Small tilt angles of 10 degrees to 30 degrees in summer and larger angles in the vicinity of 60 degrees in winter resulted in maximal productivities for these seasons. Photosynthetic efficiency was calculated for the different tilt angles for all seasons. Efficiency was low in the winter due to temperature limitations. In summer it was highest in the 90 degrees reactors, indicating that for optimal tilt angles in this season (in regards to productivity) a significant amount of radiation could not be effectively used by the culture. The results suggest a potential benefit in orientating and tilting reactors at various appropriate angles to the sun on a seasonal basis : up to 35% enhancement in annual output rate is estimated to be achievable.