Alexandrium taylorii is a harmful dinoflagellate species that is able to produce high-biomass blooms accompanied by water discoloration. These harmful events were reported in the Mediterranean basin, in selected small and sheltered beaches along the Catalan coasts and in the Balearic Islands and Sicily. In recent years the organism has been spreading over new Mediterranean areas. In 2007 the presence of A. taylorii was detected in a new beach in the northern coast of Sardinia (Italy). The bloom species caused ephemeral green–brown discoloration of the waters at Platamona beach in the summer months of 2007, 2008 and 2009, with maximum cell densities reaching 7 × 106 cells L−1 in July 2007. During bloom conditions the temperature ranged between 25 and 29°C and the chlorophyll a concentration reached very high values, ranging from 18.2 to 87.3 mg m−3. Reactive phosphorus and inorganic dissolved nitrogen were respectively between 0.04–2.21 µM P and 0.12–2.32 µM N. Our results confirmed the expansion phase of the species in the Mediterranean basin and the possibility of massive proliferation in other open beaches.