We examined the structure of rocky intertidal communities on the central Patagonian coast of Argentina. Extensive beds of the mussel Perumytilus purpuratus cover wave-exposed headlands from the low to extreme high intertidal (>95%), and a diverse assemblage of diminutive mobile invertebrates including limpets, starfish, and crabs live exclusively in the mussel bed matrix to avoid physical stress. On nearby wave-protected rocky shores, the high intertidal habitat is dominated by bare space (>85%) with mussels restricted to tide pools and crevices. Mussel beds cover the middle intertidal, while the low intertidal habitat is dominated by the erect coralline alga Corallina officanalis. These patterns are driven overwhelmingly by variation in extreme physical conditions. Desiccation stress generated by the dry southern trade winds is harsher than in any previously studied rocky intertidal system, including the Gulf of Panama, by >30% and is more severe on wave-protected than wave-exposed shores. Transplant experiments suggest that on wave-protected shores desiccation stress limits the upper distribution of mussels in the high intertidal and Corallina in the mid-intertidal, but at low intertidal elevations Corallina outcompetes mussels, restricting mussel distribution to mid-intertidal elevations. Transplant experiments also demonstrated that the coralline alga is precluded from wave-exposed shores by wave stress. Recovery from disturbance is unusually slow, ostensibly due to extreme physical stress. Consumer pressure is weak, with no common predaceous crabs or snails, and grazing by limpets showed limited control of community development, mostly by regulating ephemeral algae. Patagonian rocky shore communities are exposed to unusually harsh physical conditions and consequently are more strongly organized by physical stress than previously studied rocky intertidal communities.