189 species of seaweeds have been recorded for the Atlantic archipelago of the Azores (114 Rhodophyceae, 41 Phaeophyceae and 34 Chlorophyceae). Ten of these have been described as endemic algae. The taxonomic status of these “endemics” is far from clear, however. Studies on the relatedness of this seaweed flora to seaweed floras of surrounding areas using clustering methods indicate its intermediary position between the seaweed floras of the subtropical Macaronesian Islands (Canaries Madeira and Salvages) on the one side and those of the warm temperate Eurafrican coasts on the other side. The geographic position of the Azores is NW of the other Macaronesian Archipelagos and thus nearer to the American coast. The Gulf stream may easily transport seaweeds from the Carribean to the Azores. Nevertheless, the number of species of these islands in common with the American Atlantic coast is much lower than with, for example, the W. Mediterranean. Of one group of seaweeds, viz. algae with a strictly warm temperature distribution, not a single species occurs likewise on American coasts. This absence is probably explicable because of the narrow temperature range for survival of these seaweeds. Temperatures of surface waters around the Azores during pleistocene glaciations were not much lower than they are at present. Nevertheless, it can be supposed that a number of seaweeds now occurring in the Azores have become extinct during the last glaciation.