Mussel (Mytilus galloprovincialis) farming on submerged long-lines has recently developed off the shores of Languedoc-Roussillon, using techniques adapted to the difficult conditions of open sea sites. Annual production varies between 8,000 and 10,000 tonnes and is carried out by approximately 100 enterprises, each of which operates from 2 to 15 long-lines. Investment in equipment is considerable, about 3 million Francs for an enterprise that produces 300 tonnes per year. The estimated cost of producing one kilo of mussels is 3.7 Francs, of which 50 per cent goes on operating expenses (mostly personnel) and 50 per cent is related chiefly to depreciation. The average selling price is 4.7 Francs per kilo. Variations in these figures are mainly due to size of enterprise, production accidents (storms) and method of financing investment. The selling price is influenced by currency fluctuations in competing and client countries (Spain and Italy) and variations in output of other European mussel basins.