The migration strategy of the globally threatened Aquatic Warbler Acrocephalus paludicola needs to be known to ensure that effective action is taken to conserve its principal stopover sites. Using data from ringing stations during the autumn migration of reedbed warblers, we developed an index that allowed countries and sites to be compared in terms of their numerical importance to migrating Aquatic Warblers. According to this index, there was no significant decline in the number of Aquatic Warblers migrating through Europe during the 1990s. France appears to receive the largest numbers of this species in autumn compared with other European countries. Within France, only coastal regions are visited by the species (but not in the Mediterranean), with the largest numbers occurring in Normandy and Loire, while Brittany receives large numbers in some years but not in others. We also identified some French sites that are of major importance to migrating Aquatic Warblers depositing reserves on migration. France therefore plays a very important part in providing stopover sites for Aquatic Warblers, which means that France should play a major role in undertaking conservation measures for this threatened species.