The distribution of the toxic dinoflagellate Gymnodinium catenatum Graham in the Mediterranean Sea was once restricted to the eutrophic waters of the Albor?n Sea. In September 1999, this taxon was found for first time in the Algerian basin, being the dominant species at subsurface depths (~1 cell mL-1) associated with low salinity waters. The geographical expansion of this exotic species is not only deleterious due to its toxicity, but its recent presence is associated with changes on the structure of the Mediterranean pelagic food web.