The circadian rhythm in growth of the red macroalga Porphyra umbilicalis was investigated under different spectral light conditions in laboratory-grown thalli. A free-running rhythm was observed in constant green or red light at irradiances of 2.5 to 20 µmol m-2s-1, while arhythmicity occurred in constant blue light at 6-20 µmol m-2s-1. The circadian oscillator controlling growth rhythmicity in Porphyra uses most of the visible sunlight spectrum and possibly multiple photoreceptors with a high sensitivity for blue and a lower sensitivity for red light. This was inferred from three experimental results. (a) The free-running period, t, of the growth rhythm decreased with increasing irradiance, from approximately 25 h at 2.5 µmol m-2s-1 to 22 h at 20 µmol m-2s-1 in both red or green light. (b) Dark pulses of 3 h duration interrupting otherwise continuous green or red light caused advances during the subjective day and delays during the subjective night so that the circadian oscillator in Porphyra is able to discriminate darkness from green or red light. (c) Low-irradiance blue light pulses (2.5 µmol m-2s-1) shifted the growth rhythm in red light of higher irradiance (e.g., 10 µmol m-2s-1), and a strong, high amplitude, type 0 phase response curve was obtained that is usually observed with light pulses shifting a circadian rhythm in otherwise continuous darkness.