Abstract Lagoon environments are particularly subject to the intro-duction, either accidental or voluntary, of alien species thatmay cause biological pollution. On the basis of morpholog-ical data and molecular analyses ( rbc L and mitochondrial cox 2- cox 3 spacer sequences) we confirmed the taxonomicidentity (previously only hypothesised) of Grateloupia turu-turu in both the Lagoon of Venice and the Mar Piccolo basinof Taranto, where it was most probably introduced by shell-fish transfers. A detailed morphological description of veg-etative and reproductive thalli is given, focusing particularlyon the encrusting bases. The bases act as resting structuresthat are able to survive during unfavourable environmentalconditions. In both the Lagoon of Venice and the Mar Pic-colo of Taranto, G. turuturu does not yet exhibit invasivebehaviour. Keywords: alien species; Grateloupia turuturu ; Lagoon ofVenice; Mar Piccolo of Taranto; Mediterranean Sea. Introduction One of the major anthropogenic stresses on coastal marineenvironments is so-called ‘‘biological pollution’’, whichcomprises all possible negative effects from cell level tocommunity level linked to the introduction of alien species(Elliot 2003, Olenin et al. 2007). Recent means of transpor-tation have increased the speed of shellfish transfers foraquaculture and direct sale, facilitating the introduction ofalien species (Minchin 2007). As a consequence, coastal andespecially transitional water systems (e.g., coastal lagoons,estuaries) (Mc Lusky and Elliot 2007), which are usually thesites of aquaculture activities, are among environments mostthreatened by biological pollution.In the Mediterranean Sea, the Thau Lagoon on the Frenchcoast represents the main hot spot for the introduction ofalien species (Verlaque 2001), with 58 species of seaweedsto date (Verlaque et al. 2007). In Italy, the hot spots of marineintroductions are the Lagoon of Venice and the Mar Piccoloof Taranto basin, two transitional water systems continuouslymonitored to detect alien species as quickly as possible(Cecere and Petrocelli 2009, Sfriso et al. 2009).