The competitive interactions between two species of limpet, Patelloida alticostata (Angas), and Cellana tramoserica (Sowerby) were investigated at two sites; a low-lying intertidal platform and an adjacent subtidal reef located in southeastern Australia, during 1977 and 1982. C. tramoserica was relatively more abundant and larger in shell length than P. alticostata at the intertidal site, but P. alticostata was the more abundant species subtidaily. Fences were used to enclose various combinations of the two species to compare both intra- and inter-specific interactions. By repeating this design at two different, but adjacent sites, it was possible to test the generality of any interactions, and to investigate whether the nature of these could explain the observed differences in the relative abundances between the two sites. Intraspecific increases in density affected C. tramoserica more than P. alticostata at both sites. These effects were manifested both lethally (increased mortality at the intertidal site) and sublethally (as decreased growth and tissue weight at the subtidal site). Interspecific comparisons were asymmetrical, but were similar at both sites: C. tramoserica affected P. alticostata more than P. alticostata affected C. tramoserica. These latter effects were, however, only similar, or minor in comparison with the strong intraspecific effect that C. tramoserica had on itself. This suggests that, as has been found in some previous studies, competitive exclusion of the subordinate species (P. alticostata) is unlikely because of the greater intraspecific effects on the superior competitor (C. tramoserica). Furthermore, because there was little difference in the relative competitive abilities of the two species in the two habitats, competition between the species was probably not responsible for the different relative abundances between the two adjacent areas.