Objective. Ostreopsis ovata and Ostreopsis siamensis are tropical unicellular algae that have been found recently in the Mediterranean. Both of these dinoflagellates produce palytoxin (PTX)-like toxins that are powerful vasoconstrictors in mammals. Since 2003, Ostreopsis blooms in Italy and Spain have been accompanied by reports of respiratory problems and skin/mucosa irritation in persons in contact with toxic microalgal cells (epiphytes, plankton, or sea spray) or associated toxins. Methods. In France, a surveillance network has been set up to monitor water conditions and to protect swimmers from contamination due to Ostreopsis. Results. Between 2006 and 2009, a total of nine blooms were observed on the French Mediterranean coast including five that led to manifestations in divers, swimmers, and shoreline inhabitants. A total of 47 patients presented symptoms of involving benign or mild skin, mucosal, and/or respiratory irritation that regressed spontaneously without treatment within 12-72 h (4-12 h with nonsteroidal anti-inflammatory drugs). During the study period, five beaches were temporarily closed. Discussion. In the Mediterranean, Ostreopsis blooms induce skin and respiratory disorders when human beings are exposed to saltwater with a high concentration of algal cells. However, palytoxin dosages carried out on the food chain (urchins, mussels) indicate that this risk of toxins accumulation in seafood must be taken into account and that the surveillance network should be upgraded accordingly.