In Porphyra umbilicalis, circadian rhythms of nuclear division and growth activity persisted for at least 7 cycles in continuous white fluorescent light, with a period of 21 h for the free-running growth rhythm at 15 μmol m−2 s−1 and 10°C. Growth rhythmicity was lost at irradiances above 20 μmol m−2 s−1. The growth and mitotic rhythms seem to be driven in parallel by circadian rhythmicity, and the details of growth kinetics must be due mainly to the growth behaviour of non-dividing cells. This was inferred from the finding that individual cells continued to grow from one cell division to the next, with generation times of 2–6 days. Transfer from continuous light to 12[ratio ]12 h light[ratio ]dark synchronized the free-running growth rhythm, with a high growth peak appearing every 24 h at the start of the light phase and an increase in growth rate during the dark phase. The ascending portion of the free-running growth curve was thus shifted into the night phase of the diurnal regime and the descending portion into the light phase. This behaviour of decreasing growth activity after the morning hours, previously found in a brown and a green marine macroalga, ensures that cell activity is available primarily for photosynthesis during the day.