Halimedahas been found particularly suitable for studies of long‐distance chloroplast migration by virtue of its coenocytic structure and calcium carbonate skeleton. A circadian rhythm of chloroplast migration inHalimeda distortawas monitored by videography of segment surface pigmentation. In normal 12 h light/12 h dark treatments synchronised with dawn and dusk, the segments were green all day, began to become pale immediately the light was turned off, and then remained almost white for most of the night until beginning to re‐green a few hours before dawn. As a result of that, they were already quite green by the time the light was turned on. In continuous darkness a similar cycle, albeit with reducing amplitude and a period of about 23 hours, was maintained for at least 7 days. However, this cycle differed significantly from the normal one in that the segments did not remain green after the light was not switched on at dawn, but rather began to pale immediately thereafter. Conversely, in continuous light the segments did not become pale at any time. Thus, the rhythmical re‐emergence of the chloroplasts before dawn and their subsequent withdrawal appears to be controlled by an endogenous rhythm which is independent of light. However, light does completely, but reversibly, inhibit the chloroplast withdrawal component of the cycle. This behaviour of the chloroplasts inHalimedais very similar to that in the related alga,Caulerpa, but it is quite different from that in another extensively Studied but unrelated siphonous green alga,Acetabularia, in which the circadian rhythm of chloroplast migration is maintained in continuous light.