In order to strengthen and adapt an initial monitoring system established along the French Mediterranean coast during the summer of 2007, it was necessary to review the available knowledge on the effects of Ostreopsis ovata, using a public health risk assessment approach. O. ovata is a single-celled alga increasingly reported in the Mediterranean, which synthesizes a toxin called palytoxin. This alga is responsible for several clusters of cases of irritation, accompanied by coughing, fever, and respiratory problems, described in Europe since 2002. It is currently not possible to determine whether these health effects occurring after inhalation of aerosols are attributable to palytoxin. Furthermore, ingestion of fish or shellfish flesh can lead to severe poisoning. No cases of food poisoning have yet been reported in Europe. It is not possible to establish a dose-response relationship based on the available data. This is why the concentration threshold for this microalga in water currently set to trigger public health intervention is based on an empirical approach. Nor is it currently possible to link population exposure to either a quantity of microalgae or a concentration of palytoxin. However, the conditions under which blooms occur and the situations that increase population exposure are now well described. Ultimately, the data are still insufficient to arrive at a quantified risk assessment. Further research is still needed, on the one hand, to determine the factors underlying the occurrence of blooms in order to identify at-risk areas and periods, and, on the other hand, to develop bloom forecasting tools capable of establishing a sampling strategy useful for more appropriate monitoring, targeted at areas of human exposure. (R.A.)