The aim of this study was to prove the presence of the dinoflagellate genus Ostreopsis along the Istrian coast of the Adriatic Sea and investigate its toxicity. The research was conducted at a public beach north of Rovinj, in the period from the 22nd September 2010. until the 27th October 2010., during the maximal recorded abundance of the genus Ostreopsis. Algae of the genus Ostreopsis are epiphytic. They require a stable habitat and settlement base because adverse weather conditions might cause a separation of cells from the substrate. We followed temperature, salinity, weather conditions and abundance of Ostreopsis sp. on the macroalgae Cystoseira crinita, which is a perfect substrate for the biofilm produced by the genus Ostreopsis. This research allowede us better insights into the ecology of this genus. Monitoring the dynamics of this bloom showed the importance of environmental factors for the development and completion of a bloom event, especially the hydrodynamic conditions such as wind, waves and currents. We recorded the toxins palytoxin and ovatoxin. Our analysis showed that their production was extremely high during this natural bloom.