Macrobenthos communities were studied on fringing reefs of the An Thoi Islands using scuba-diving techniques. The islands are located in the turbid and highly eutrophic waters of the eastern Gulf of Siam. We studied species composition and population densities and biomasses in common species of algae, coelenterates, mollusks, and echinoderms and also the degree of substrate coverage by macrophytes and corals. A pronounced vertical zonation is revealed in the spatial succession of different macrobenthos communities. The domination of massive porites in almost all reefs of the Gulf of Siam is due to their capability to survive under conditions that are stressful for many corals. They predominate over other scleractinians in terms of the productivity of organic matter, degree of substrate coverage, and species diversity. They also constitute the reef skeleton and play a significant role in the expansion of the reef area in the muddy bottoms of the Gulf of Siam.