Twelve survey cruises on the continental shelf offshore of Ubatuba, southeastern Brazil, were conducted during 1985 to 1987 as a part of a broad oceanographic investigation on the tropical Atlantic shelf ecosystem. Two different water masses influence the seabed: South Atlantic Central Water (SACW) and Coastal Water (CW). The first penetrates towards the coast during summer months, and retreats offshore in the winter. Distribution, density and biomass of the benthic megafauna show 2 domains: inner shelf and outer shelf, both separated by a front. High abundance of the brachyuran crab Portunus spinicarpus characterizes the frontal system and occasionally changes the structure of the inner and outer shelf assemblages. Mulhvariate analyses showed that water masses and sea depth are the main factors structuring the benthic megafauna comn~unities. Clustering analysis disclosed 4 dlstinct faunistic assemblages: (1) coastal communities withln the CW domain, (2) inner shelf communities influenced by a mixed zone with CW predominant, (3) frontal zone communities where SACW was predominant, (4) outer shelf communities within the SACW domain. Species diversity was almost hon~ogeneous on the entire shelf except in the frontal zone where it sharply decreased. The common species, both perenial and seasonal, showed a consistent pattern of d o m a n c e through the study period. Adjustments occur within the community whereby some species increase in abundance when others decrease, maintaining a stable pattern. A 'cyclic' stability-adjustment mechanism is proposed to explain the dynamics of the megabenthos in the Ubatuba region.