The depth of the Posidonia beds nearly always depends on the transparency of the coastal waters. Any change in this factor, for example due to eutrophication, results in an attenuation of the amount of light received by the deepest beds, which hinders the seagrass growth in certain areas of the Cote d'' Azur [France]. Ancient beds which were deeper than 30 m are now reaching an average depth of only 25 m on the southeastern French coast. Using the facilities made available by the special units of the French Navy Triton and Griffon (submarine), a method was perfected enabling them to trace with accuracy the lowest limit of the Posidonia beds. This method was used during the POSEIDON 76 campaign, when more than 23 km of the deepest part of the beds found in the Alpes-Maritimes (France) coastal areas, were traced on 5 maps each on the scale of 1/5000. The data collected on board the submarine and also during many explorative dives in the western Mediterranean Sea, describe 3 different types with regard to the deepest levels of the beds. The density of leaves, the position of rhizomes and the thickness of the layer of rhizomes enable the authors to classify 3 types. Type 1 corresponds to beds of feeble density (sometimes covering less than 5% of the whole bottom surface) the rhizomes of which are in a horizontal position. Type 2 corresponds to beds of high density (covering up to more than 50%) the rhizomes of which are in a vertical position. These rhizomes do not make up a thick layer (< 20 cm). Type 3 corresponds to beds of very high density (covering up to more than 75% of the bottom surface) the rhizomes of which, in a vertical position, make up a thick layer (sometimes more than one meter). Deep beds may exhibit a lower boundary under the 3 types described above. The assumptions concerning the reasons for this halt in growth are discussed in this paper. A description of the 3 types of beds, the growth rate of which is particularly poor, is also given.