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Dictyota is a brown alga genus inhabiting tropical to warm temperate environments where it performs an important ecological role, such as a food source, shelter and substrate to several species of invertebrates and other algae. The taxonomy of this genus is troublesome, with poor species delimitation and doubtful records in the literature. Dictyota dichotoma, the type speciesof the genus, was commonly accepted as a cosmopolitan species because of the inaccurate geographical distribution range as a consequence of its misidentification in several parts of the world. Recent studies with molecular data revealed new species and a new understanding of the evolution and biogeography of the genus Dictyota. These studies confined populations of D.dichotoma to Europe (Atlantic and Mediterranean coasts) and the Macaronesian Islands (Azores, Madeira and Canary archipelagos). Also, they confirmed its presence in South Africa, but, whether the species was native or introduced in South Africa could not be verified. In the present study two regions of cpDNA (psbA, rbcL) and one region of mtDNA (nad1) from Argentine samples, identified morphologically as D. dichotoma, were analyzed and compared with other Dictyota species. The identity of these samples as D. dichotoma was confirmed. A haplotype network analysis using all available psbA sequences distinguished seven haplotypes divided into two geographic groups: Atlantic-Mediterranean and Canarian. In Argentina and South Africa, only the most common haplotype of the Atlantic?Mediterranean group was observed. According to the paleoceanographic currents and the presence of European haplotype, the introduced nature of D.dichotoma is indicated. Fil: Pereira Lopes Filho, Erick Alves. Universidade Federal do Estado do Rio de Janeiro; Brasil Fil: Salgueiro, Fabiano. Universidade Federal do Estado do Rio de Janeiro; Brasil Fil: Mattos Nascimento, Silvia. Universidade Federal do Estado do Rio de Janeiro; Brasil Fil: Gauna, Maria Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina Fil: Parodi, Elisa Rosalia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina Fil: Campos De Paula, Joel. Universidade Federal do Estado do Rio de Janeiro; Brasil