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A detailed study of vegetative and reproductive phases of several populations of a common species of Ulva from Thau Lagoon (France, Mediterranean Sea) showed that the material is in good agreement with Asiatic Ulva pertusa Kjellman. Since October 1994, the date of its first observation in Thau Lagoon, U. pertusa has developed abundant reproductive populations. It is highly probable that this species was introduced at Thau along with oyster transfers from the Pacific. Ulva pertusa is new to the Mediterranean Sea.