A new species of the dinoflagellate genus Alexandrium, A. tamutum sp. nov., is described based on the results of morphological and phylogenetic studies carried out on strains isolated from two sites in the Mediterranean Sea: the Gulf of Trieste (northern Adriatic Sea) and the Gulf of Naples (central Tyrrhenian Sea). Vegetative cells were examined in light and scanning electron microscopy and resting cysts were obtained by crossing strains of opposite mating type. Alexandrium tamutum is a small-sized species, resembling A. minutum in its small size, the rounded-elliptical shape and the morphology of its cyst. The main diagnostic character of the new species is a relatively wide and large 6th precingular plate (6), whereas that of A. minutum is much narrower and smaller. Contrary to A. minutum, A. tamutum strains did not produce PSP toxins. Phylogenies inferred from the nuclear SSU rDNA and the D1/D2 domains of the LSU nuclear rDNA of five strains of A. tamutum and numerous strains of other Alexandrium species showed that A. tamutum strains clustered in a well supported clade, distinct from A. minutum.