When S. platensis media were irradiated with the intermittent light having a periodicity of 0.0044-0.60 s(-1), the oxygen production ratio increased with an increase in light periodicity. The value of the oxygen production ratio is greater than one when the irradiation time during a cycle is shorter than 1 s. Under these conditions, the oxygen production ratio increases with increasing dark time. The effect of the intermittent light on the oxygen production rate of S. platensis was found to be qualitatively described by the simple model which combined the oxygen production rate of S. platensis and the respiration rate.