Ecological research on the faunas of the sheltered beaches of the region of Arcachon. In these ecological studies of the sheltered beaches of the bay of Arcachon (France), the author distinguishes and locates four traditional zones: slikke, upper slikke, schorre and parhalian zone. Animal communities and edaphic structures of each zone are treated in detail. In the slikke, four fundamentaI facies are distinguished: Zostera nana zones, bare sands (such as Arenicola sands), Nereis diversicolor slime and Scrobicularia slime. Each facies is analyzed with regard to its amount of water, porosity, salinity, pH and dissolved oxygen content, as well as its hypsometric distribution. With respect to the fauna of sand and slime slikkes, the importance of the endogeous macrofauna, its infralittora] characteristics and the low degree of mobility of its species are emphasized. In the upper slikke five facies are described: Bledius spectabilis clay sands, Corophiurn volutator slimy sands, Bledius arenarius porous sands, Spartina maritima areas and micro cliffs. Again, abiotic characteristics and animal communities of each facies are studied successively. The importance of edaphic structures for the distribution of animal communities, and vice versa, is stressed. For instance, the amphipod Corophium volutator and the coleopteran Bledius spectabilis build tubes with firm and permanent inner surfaces only in certain types of sediment: yet, while the Bledius spectabilis tubes are dense and watertight, those of Corophium volutator are tess tight and o~en flooded. Limites externes des stikkes et des schorres. Zone parhalienne Du c6td inf&ieur, la slikke est limit& sans conteste au bord du chenai ou lit mineur de Ia rivi~re. Elle inclut, lorsqu'il existe, l'herbier de Zostera nana (MAssART 1907/1908). Du c6td supErieur, la limite continentale du schorre est beaucoup plus difficile &ablir. FRANCls-Bo~ur (1947), consid~re ie schorre comme un ensemble morphologique, d'origine g6ologique, pouvant inclure des terrains qui ne sont plus actuellement inondables par la mar&, par exemple les ouldjas cultivabtes de l'estuaire du Bou-Regreg au Maroc. Comme l'a soulign6 JACQOET (1949), si cette opinion est soutenable aux regards d'un gdographe, il ne saurait en &re de m4me ~ ceux d'un 6cologiste. VEI~Grt~ (1956) a tr~s justement oppose les s&orres littoraux aux schorres d'estuaires. Dans les s&orres littoraux, le crit6re bathym&rique colncide avec la mar& de salinit6; un accident topographique, digue, cordon littoral, ro&e en place, marque topographiquement la limite des hautes eaux; cet accident s'accompagne alors de la pr& sence d'nn niveau bionomique original de faible extension, la zone parhalienne, qui marque nettement le passage des formations halophiles aux formations continentales. Dans les s&orres d'estuaires, la rupture morphologique de la pente de la plage, lorsqu'elle existe, est gEn6ratement moins accusde; en outre, ies fortes mar&s s'accompagnent encore d'inondations, mais l'eau recouvrante est une eau presque douce, provenant des dEb...