Culture experiments designed to test the apparent correlation between the seasonal behaviour of Desmarestia aculeata and light conditions in the sea suggest that light intensity and daylength interact in their effect on development and growth, and that it is the total quantity of light energy received per day that is important in this resptect. Maturation of gametophytes took place between 54 lx and 1,540 lx mean daily illuminance (MDI) but was inhibited at lower levels. Mean daily . The rate of maturation of gametophytes increased with increasing MDI up to 615 lx. Similarly, the developmental rate of young sporophytes increased with increasing MDI, but there was no light saturation effect up to the maximum of 1,540 lx tested. The ability of gametophytes to produce young sporophytes was not impaired by a period of 93 days at a very low light intensity, since sporophytes developed at once with increased illumination. Marked elongation of regenerating branches on mature sporophytes took place at the low MDI of 33 lx.