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Two species of the green alga genus Caulerpa are considered as invasive in Tunisia: Caulerpa racemosa and C. taxifolia. C. racemosa is found in a many new localities at the central and southern Tunisian coast, and even in the North of the country in colder water. These new observations lead to the hypothesis of the multiplicity of the origins of the introduction of the species and suggest possible invasive potentialities. The general thallus, and more particularly the number, the shape as well as the arrangement of ramelli on fronds allowed us to identify four morphologically different groups. These groups could be either different systematic entities or polymorphic variations of the same taxon due to different ecological conditions. Caulerpa taxifolia was ob- served for the first time in Tunisia at the roadstead of Sousse in the beginning of the year 2000. It is recognized as identical to that found in the other Mediterranean localities (GenBank number AJ228960). Up to the end of March 2000, a submarine prospecting campaign revealed an affected area of about 350 ha in front of 2 km of coast. Colonies of Caulerpa taxifolia disseminating in this area appear in isolated spots. form, In the growing stage, they form surfaces varying be- tween 0.5 to 6 m 2 and located 20 to 100 m from each other. The total covered surface was estimated to be between 0.5 and 1 ha. [ABSTRACT FROM AUTHOR]