A potentially toxic benthic dinoflagellate microalga of the genus Ostreopsis has recently proliferated in the northern Mediterranean. Inhalation of contaminated sea spray was responsible for an outbreak in Genoa in 2005, resulting in 225 people seeking emergency care and 20 hospitalizations (febrile respiratory syndromes and skin irritations). In September 2006 in Marseille, swimmers and divers visiting a cove on the Frioul Islands exhibited symptoms associated with Ostreopsis. Since 2007, the Directorate General of Health has implemented a system combining epidemiological and environmental surveillance, as well as preventive risk management related to the presence of Ostreopsis in bathing waters. This system covers the Mediterranean coastline and involves partners such as Ifremer, the Marseille Poison Control Center, the Regional Health Agencies, and the Interregional Epidemiology Units in Marseille and Montpellier. A decision-support unit, comprising experts, risk assessors, and risk managers, proposes preventive measures to the mayors and prefects concerned. Given the continuing uncertainty regarding the causes of Ostreopsis blooms (massive algal blooms) and the concentration levels of this algae in seawater that may pose a health risk, this exploratory surveillance evolved between 2007 and 2010. In the absence of mass febrile respiratory syndromes linked to exposure to sea spray contaminated with Ostreopsis, the symptoms observed in connection with Ostreopsis proved to be mild and comparable to those observed with jellyfish. Beach monitoring was combined with research efforts, particularly regarding food safety risks (bioaccumulation of palytoxin and its derivatives in seafood, especially sea urchins). (R.A.)