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 CO 2 and were subjected to different light intensities in the range of 75–1000μEm −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 1000μEm −2 s −1 . The light intensity per cell required for the high specific growth rate of 0.5day −1 was determined to be 28–45μEg-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 (250–1000μEm −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.