Extended periods of growth under continuous light (> 10 generations) effectively removed the normally strong circadian controls on the growth ofChlamydomonas reinhardii.Selection‐synchronized cultures of such organisms showed the cell cycle time to be 20h and there was no detectable circadian rhythm in chlorophyll a accumulation. However, an ultradian rhythm of period length 55 min in the level of chlorophyll a similar to that previously reported for LD cultures was still observed. These results provide further evidence for the importance of intracellular timing independent of environmental influences.