The dinoflagellate Ostreopsis siamensis was studied as part of a broad national monitoring program along the Lebanese coastal waters. The monthly and spatial variations of its density in water samples were carried out at 4 different rocky stations from September 2000 to December 2001 and for a longer period (April 1997-December 2001 and January 2010 to December 2010) in one of the stations. Results showed that O. siamensis is present almost in all rocky shores along the Lebanese coastline from south to north between May and November when the surface seawater temperature ranges from 23 to 30.5 °C, with a maximum in abundance between July and September. In 2001, the population densities reached 10 500 cells/L in July and 5 700 cells/L in September at two stations both located in the southern coast. In 2010, the analysis of larger water volumes revealed the presence of this species in February, March and April; however densities were lower than 1-3 cells/L. Significant positive correlation was observed (P<0.01) between the occurrence of the species and nitrate concentrations, whereas the correlation was negative with orthophosphates. Our results suggest that O. siamensis is a thermophilic species. It constitutes a permanent component of the microalgal flora of Lebanese waters and its abundance depends on water enrichment.