In spite of the controversy surrounding the accidental introduction of the Japanese seaweed Sargassutn muticum (Yendo) Fensholt onto two continents, the startling south-ward expansion of its range on the Pacific coast of North America (Norton, 1977 a), and the failure of an eradication campaign on the south coast of England, surprisingly little is known about the ecology of this species. Work has recently begun in both southern California and Britain (Nicholson et al. 1977; Lewey & Farnham, 1977). Unfortunately the ecological studies in Britain have been hampered by the conflicting aims of trying to study the plant and at the same time trying to eradicate it.