The prosobranch limpet Cellana tramoserica grazes on microalgae, including the spores of macroalgae, and coexists with the pulmonate limpets Siphonaria denticulata and S. virgulata at mid-tidal levels on sandstone shores in New South Wales. These siphonarians graze on macroalgae, leaving the basal parts of the thalli intact. Where Cellana graze, they are capable of removing all algae; where Siphonaria graze, they leave at least a thin film of alga on the rocks, which is available to Cellana. S. denticulata normally show invariant homing behaviour, whereas S. virgulata tend to move around at random when Cellana are present, but apparently home when Cellana are absent. Both siphonarians have been observed to show opportunistic behavioural responses by moving towards patches of macroalgal foods when they become available. Experimental ecclosures of limpets at different densities and in different combinations revealed that Cellana tramoserica suffered increased mortality and reduced growth due to intraspecific competition when at increased densities. There was no effect on Cellana of increased densities of either species of Siphonaria. Nor was there any interspecific interaction between the siphonarians. Both species of Siphonaria showed some reduction of growth at increased intraspecific density. More importantly, both showed increased mortality when enclosed with low densities of Cellana. Larger densities of Cellana had no effect; the numbers of Cellana could not be maintained because of the reductions caused by intraspecific competition. Even after 27 weeks in enclosures with Cellana, the numbers of Siphonaria never declined to zero in any experimental enclosure. Thus, Cellana has a competitive effect on the survival of siphonarian limpets, but is unable to exclude them from an area of the shore. Siphonaria spp., in contrast, have no effect on Cellana. The nature of the competitive interactions between these types of limpets is explaied in terms of their methods of feeding; Cellana can exploit the food-resource before it reaches a suitable size for Siphonaria. The coexistence of Siphonaria spp. with Cellana is discussed with respect to the behaviour of the pulmonates. Intraspecific competition leading to reduced densitities of Cellana, however, will ensure that Cellana cannot exploit all the food resources, and some will be available to Siphonaria. The consequences of inter- and intra-specific competition among grazing gastropods are discussed with reference to the structure of intertidal communities, and it appears that competition for food is fundamentally different from competitive interactions for space in the organization of such communities.