The natural mortality of oysters could by considered, in a man controlled ecosystem, as the result of the action of biological factors, and human factors other then the harvest of the population. Such mortalities need to be estimated in order to be used in population dynamics, as a tool for management purposes. ln Marennes-Oleron bay, different ways of estimating the natural mortality are available. Several oyster farms were monitored for assessing growth, mortality and production in rearing conditions. Furthermore, an experimental framework which includes 15 stations gave an appraisal of the same factors under controlled conditions. Finally, another estimate will be soon available by comparin9 the number of recruited oysters, and the number of oyster available for commercialisation, which were effectively reared within the bay. The seasonal mortality was described and its implications in terms of population dynamics were discussed.