To determine the optimum light intensity per cell required for rapid growth regardless of cell density, continuous cultures of the microalga Chlorella zofingiensis were grown with a sufficient supply of nutrients and CO2 and were subjected to different light intensities in the range of 75-1000 μE m(-2) s(-1). The cell density of culture increased over time for all light conditions except for the early stage of the high light condition of 1000 μE m(-2) s(-1). The light intensity per cell required for the high specific growth rate of 0.5 day(-1) was determined to be 28-45 μE g-ds(-1) s(-1). The specific growth rate was significantly correlated to light intensity (y=0.721×x/(66.98+x), r(2)=0.85, p<0.05). A high specific growth rate was maintained over a range of light intensities (250-1000 μE m(-2) s(-1)). This range of light intensities suggested that effective production of C. zofingiensis can be maintained outdoors under strong light by using the optimum specific light intensity.