ABSTRACT. Here, we present the first molecular evidence that the trop~cal green alga Caulerpa taxifolia, which is quickly spreading in the Mediterranean and out-competing natlve species, escaped to the sea from a public or private equarium. Our data show that this alga is genetically identical to the strain cultivated in western European aqudria since the early 1970s The facillty with which this strain is obtained world-wide represents a potential danger of additional biological invasions.