Abstract.A large‐scale map of thePosidonia oceanicaseagrass beds as well as other benthic substrata and communities was compiled for a cove of a marine reserve free from pollution and man‐made disturbance. An extensive diving survey of the cove and the interpretation of aerial colour photographs were used. The greater part of the cove is occupied by theP. oceanicameadow. Patches of dead “matte” were observed at a depth of between 7 and 14 m. They may be the result of a natural hydrodynamic equilibrium between building and