The present study forms part of a more extensive one on the ecological factors which govern the distribution of the microfauna inhabiting seaweeds on rocky sea-coasts. This part of marine ecology is lagging far behind other branches. I wish to stress two points in particular: fasirst, that our understanding of the composition of the littoral (intertidal) fauna would increase very much if this fauna can be linked with the true infralittoral fauna, and, secondly, problems involving the synecological aspects of marine biology cannot be approached before more is known about the autecology of the animals composing the cryptofauna (i.e. those living in the shelter of algae) and the causal factors determining their occurrence and distribution within a given area under stated conditions. On this point the work of T. A. Stephenson is especially relevant: as a result of his extensive investigations on the composition of the intertidal flora and macrofauna he has come to stress the importance of autecological rather than of biocenological work on the shore (Stephenson et al. 1942; Stephenson, T. A. & A., 1949). Moreover, he has clarified the intertidal terminology and introduced the most useful terms “supra-” and ‘infralittoral fringe”. Since his terminology appears to be the best suited to meet the needs of ecologists, I hope it will be generally applied in future. Most work on littoral ecology has been limited to macro-organisms, valuable data on the ecology of microfaunas of seaweeds have been supplied by Colman (1940), Kitching, Macan & Gilson (1934), and Dahl (1948).