Distribution patterns of the barnacles Chthamalus dentatus Krauss, Tetraclita serrata Darwin and Octomeris angulosa Sowerby were studied along the rugged and exposed shoreline of Transkei (Southeast Africa). The data argue in favour of physical rather than biological control of community structure. A strong correlation exists between species distribution and two major physical parameters, namely emergence1submergence (EIS) regime and the degree of exposure to wave impact. A two-dimensional conceptual model was derived, in which the habitat of each species is delimited by specific combinations of the two parameters. Thus, C. dentatus occurs under virtually any EIS regime where wave impact is low. 0. angulosa in turn occurs where EIS ratios are low, but where wave impact is intermediate to high, whereas T. serrata occupies an intermediate position in terms of both parameters. Although individual habitats overlap over a limited range of physical conditions, there was no evidence that competition controls the distribution or zonation of these species. The model is consistent with the distribution patterns observed along the entire Southeast African coastline, and amongst other things, also explains the occurrence of zonation patterns. Circumstantial evidence suggests that the segregative process occurs after the larvae arrive on the shore, but prior to metamorphosis. The model may have implications for other intertidal organisms, including algae.