This work forms part of a general study concerning the geology and bionomy of the coral reefs of Mascarene Archipelago (Reunion, Mauritius, Rodriguez), and gives the results of geomorphological and ecological investigations relating of the fringing jbef of Rodriguez Island.

The coral structures form a peripheric and ahnost continuous girdle measuring 90 km in length, 50 m to 8 km in width, with a shallow lagoon (0.5 to 4 m at L.W.S.).

From the top to-10 m, the seaward slope presents three ' horizons'; the upper one with Millepora, Acropora, Seriatoporidae ; the middle one with Echinopora, Echinophyllia; and the lower one with Porltes, Fayta, Synarea, Fungia.

The outer reef-flat is composed of spurs and grooves covered with Madrepores and Alcyonaria (30%) and calcareous algae (70%). Behind this zone, there is a flat (' trottoir r6sifal') with a rich coral fauna (Pocilloporidae, Stylophora, Montipora, Pavotia, Goniastrea, Acropora, Leptoria, Galaxea), Alcyonaria (Lobophytum, Sarcopkytum) and Zoantharia (Palythoa). In front of dominant currents ESE-SE, a boulder rampart is built, more or less cemented by Melobesiae; near the back-reef areas, this formation supplies some layers of coral gravels. Behind the rampart is an inner flat with Heliopora, Goniastrea, Leptoria, Platygyra, Echinopora, more or less structured ; a bay-flat cut by sandy channels reaching the' lagoon', the bays become scattered and give a sparse inner flat,b eing distinguished by the Pheophyceae, Turbinaria omata. The last feature of the inner flat shows a microato Uarea with Porites cf. somaliensis.

The ' lagoon' presents a skeletal carbonate sandy bottom, scattered with sparse coral patches and coral-heads. Due to hypersedimentation, a really sandy bottom only remains in process of emergence and locally settled with beds of marine Phanerogams (Halophila ovalis and H. stipulacca). These grasses are restricted to the coast line and the lagoon sector.