Summary The so-called dark arrangement of the phaeoplasts of the brown alga Dictyota dichotoma is different from both low-intensity and high-intensity arrangement. After several hours of darkness the phaeoplasts occupy the anticlinal and the inner periclinal cell walls, whereas the outer periclinal walls are nearly free from phaeoplasts. At the inner periclinal walls far more plastids are found above the lumina than above the walls of the large medullary cells. This omedullary cell pattern» disappears when the opposite cortical layer of the thallus is removed and the medullary cells are destroyed by dissecting the thallus longitudinally. However, the polarity between the oouter» and oinner» parts of the cell with respect to phaeoplast arrangement is not eliminated by this manipulation. Since the dark arrangement looks like an intermediate state between high- and low-intensity arrangement, its transmittance value lies between those of the other two arrangements. Irradiance-response curves of the movement from low-intensity to dark arrangement and vice versa revealed that movement towards the dark position starts at about 0.5 Wm−2 and is complete at about 2 × 10-4 Wm-2. Lower irradiances cause no further change of the arrangement. The speed of the movement is the faster the longer the pre-irradiation with low intensities lasts. By the transfer from light to darkness circadian rhythms are initiated.