There have been numerous investigations into the patterns of zonation of intertidal organisms on temperate coasts, and these have been extensively reviewed (Stephenson & Stephenson 1949; Doty 1957; Southward 1958; Lewis 1964; Newell 1970). Earlier surveys which were related to tidal levels on the shore (e.g. Smith & Newell 1955; Williams 1964a, b, 1965) can be compared only with some difficulty. In addition, many earlier surveys were over relatively short periods, population densities were not recorded, and only the extreme vertical limits of the species were accurately determined. A reinvestigation of the intertidal zonation of common prosobranchs and intertidal algae in the Plymouth region was therefore carried out to form a basis for studies on larval settlement, histological investigations of annual reproductive cycles and experimental analyses of the behaviour of the adult animals in a laboratory tide model, which have been described elsewhere (Underwood 1971, 1972a, b, c). It was considered essential that the survey described in this paper should be related to Ordnance Datum (OD) Newlyn, so that the data would be readily available for comparison with subsequent observations on other shores. The present surveys were compared with previously described British transects in an attempt to estimate how far the patterns of zonation discovered could be directly related to tidal rise and fall. It has been considered in many general treatises on intertidal ecology that the zonation of littoral animals and plants is primarily caused by the tidal cycle (e.g. Southward 1958, p. 161; Lewis 1964, p. 8). Stephenson & Stephenson (1949) have seriously questioned such an idea, pointing out that vertical zonation may be oserved in land-locked lakes remote from typical marine tidal fluctuations and, in fact, wherever there is an interface between a body of water and air. It was hoped that the present observations in the neighbourhood of Plymouth would discover whether the patterns of zonation would show seasonal variations, which might be unrelated to tidal changes. A weakness of previous detailed investigations of intertidal population structures and growth-rates over long periods has been that large samples have been removed from the shore and counted and measured in the laboratory. Subsequent samples, from the same position on the shore, would therefore not necessarily contain the same proportions of size-classes and would not relate to undisturbed populations. In the present survey, the numbers of each of three species of trochid, Monodonta lineata (da Costa), Gibbula umbilicalis (da Costa) and G. cineraria (L.), and of two species * Dedicated to Mr R. Bassindale on his retirement from the Department of Zoology, University of Bristol. f Present address: Department of Zoology, School of Biological Sciences, University of Sydney, N.S.W. 2006, Australia.