In this paper we report the first finding of the hydromedusa Gonionemus vertens Agassiz, 1862 in the southern hemisphere. About 30 newly released medusae were found within an aquarium on September 2008. The aquarium contained benthic samples collected in intertidal and subtidal rocky fringe off Mar del Plata, near a commercially important harbor in Argentina. Medusae were fed with Artemia salina until sexual maturation. Possible way of species introduction is discussed.