The rate of oxygen evolution of the tropical red alga Kappaphycus alvarezii was measured for 6 days in the laboratory using a computer-aided method for long-term recording. In cool white light Kappaphacus exhibited a robust circadian rhythm of O2 evolution in the irradiance range of 100 to 1000 µmol m-2 s-1. With increasing irradiance the period of the free-running rhythm, tau, decreased in blue and increased in red light, but did not change significantly in green light. The accelerating or slowing action of blue or red light, respectively, points to two photoreceptors used in the light transduction pathway of the circadian oscillator controlling oxygen evolution or the light reactions of photosynthesis in Kappaphycus. No significant changes of tau were observed with increasing irradiance in cool white light, possibly due to the additive, opposing responses caused by blue and red light.