In recent years much information has accumulated on the composition and distribution of algal populations in the Isle of Man, with increased knowledge of their ecological relationships (Knight & Parke, 1931, 1950; Jones, 1948; Lodge, 1948,1954; Burrows & Lodge, 1949, 1951; Southward, 1953). This, however, has mostly come from studies made in the intertidal region, and even here many problems remain for which, at present, no satisfactory explanation can be found. The mostdimcult of these, perhaps, relates to the appearance of algal species for a limited period of the year only: it is not known where and in what form their life-histories are continued for the rest of the year. The littoral and sublittoral regions are known to have many species in common. This means that the differences between the regions, as environments for the growth of algae, can be bridged by the plasticity of the species concerned. It seemed likely, therefore, that a detailed study of the sublittoral algal populations might help towards an understanding of the ecology of both regions and also of the interactions between them.