1. The work concerning the ecology of free-living marine algae is reviewed briefly. 2. The species examined in this investigation are the PhaeophyceaeEctocarpus fasciculatus, E. siliculosus andPilayella littoralis. 3. From culture experiments using excised filament fragments as inoculum it is shown that these species can regenerate new tissue from detached pieces. 4. The longevity of the free-living plants in nature is assessed using the apparatus illustrated in Figures 1 and 2 and found to be in the region of two months. 5. Free-living plants may propagate vegetatively by fragmentation but they also continue to produce unilocular and plurilocular sporangia while in this condition. The fertility of the free-living plants is usually lower than in attached plants. 6. The main morphological effects of the free-living state are increase in angle of branch emergence, reduction in length of lateral branches and loss of branch pattern. This is accompanied by reduction in size of reproductive bodies. It is shown thatE. distortus andE. landsburgii constitute free-living ecads ofE. fasciculatus. 7. The role of free-livingEctocarpus andPilayella in the entire species populations is thought to be minor but it is suggested that they might form a sublittoral reservoir of material in summer when intertidal conditions may be exacting.