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Notes: Summary Previous investigations have shown that competition for space among sessile organisms in rocky intertidal communities is often reduced by predation or harsh environmental factors. Grazing gastropods are unlikely to compete for space, but at high densities might compete for food, unless some factor, such as predation, reduced the densities. The coexisting species of herbivourous gastropods on rock-platforms in New South Wales are not, however, subject to high levels of predation on shores sheltered from waveaction. In this study, three of these species were caged at densities from the natural, to 4 times the natural density on the shore, in different combinations of species, to investigate their competitive interactions. All three species, Nerita atramentosa, Bembicium nanum and the limpet Cellana tramoserica, showed increased mortality and reduced weight at increased density, over 100- or 200-day periods. The effect of high density on Cellana was greater than on Nerita or on Bembicium. In addition, Nerita was competitively superior to the other two species. Cellana, at high densities, adversely affected Bembicium. Nerita was not affected by high densities of either of the other two species, and Cellana was not affected by Bembicium. Under entirely natural conditions, the effects of increased density of Cellana on the mortality and tissue-weight of Bembicium could not occur, because of the high rate of mortality of Cellana when at increased density of its own species. The other effects of increased density of snails would, however, occur. Thus, there can be regulation of numbers of snails because of high densities of their own or other species on the shore. The supply of benthic, microalgal food is proposed as the limiting resource for which the species compete. Hypotheses are proposed to account for the mechanisms by which the three species acquire different amounts of the resource. These are based on aspects of the feeding biology of the snails. The continued coexistence, in intertidal regions, of Cellana, which shows the highest rate of mortality due to members of its own species, with the competitively dominant Nerita, which increases the mortality of Cellana, is apparently due to three factors. These are: (i) the presence of subtidal refuges for breeding populations of Cellana, (ii) regular spatial dispersion of Cellana which would probably decrease intraspecific competition for food, and (iii) the high variability in space and time of recruitment of planktonic larvae of both Cellana and Nerita. This last factor makes it unlikely that high densities of Nerita could occur on all shores in every year. Cellana could always recolonize any area where its density had previously been reduced, and thus, cannot be excluded by competitive interactions. Competitive interactions at high densities of gastropods can therefore cause reductions in the number of each species present on a shore. They cannot, however, lead to exclusion of any species or alter the composition of the community of herbivores on the shore. The difference between competition for space by sessile intertidal organisms, and competition for food by gastropods which graze on microalgae is discussed with respect to the structure of intertidal communities. In the former case, competitively dominant species must be dislodged from the resource, space, by disturbance (e.g. predation or hazards) before recolonization by inferior competitors can occur. In the latter situation, the food resource is renewable without further perturbation of the community, and the competitively dominant species could never consume it completely over a wide enough area, and for a sufficient length of time, to eliminate totally another species.