The aim of this paper is to evaluate the colonization of the green alga Caulerpa racemosa var. cylindracea thirteen years after it was signalled in the Mediterranean Sea. The alga has been recorded along the coasts of 11 nations, developing on all kind of substrata, both in polluted and in unpolluted areas, between 0 and 70 m depth. The length of coastline affected by the invasion of C. racemosa var. cylindracea in Spain, France, Italy and Croatia ranged from 700 to 750 km at the end of 2003. The main aspects of the ecology of the species in the Mediterranean Sea and the effects of the invasion on human activities in coastal areas are also discussed.