The Ranee basin, on the northern coast of Brittany, is the only site where a full-scale evaluation of the ecological impact of a tidal power scheme, after 20 years of operation, has been made. The isolation of the estuary, during the construction phase, was particularly damaging to the environment. Gradually, after the scheme was put into service, an increasingly diverse flora and fauna became established. The patterns of distribution of this flora and fauna, their groupings into ecological units and the nature of their interrelationships, indicate a variable degree of biological adjustment to the new environmental conditions. Migratory organisms are able to pass via sluice gates and turbines. However, the new ecological equilibrium, established in the space of 10 years, remains fragile and, being linked to the degree of stability of abiotic conditions, depends to a large extent on the operating conditions of the power station.