A survey of planktonic dinoflagellates and related cysts was carried out in the Gulf of Trieste throughout one year from April 1992 to March 1993. 113 taxa were recovered by the analysis of phytoplankton net samples. The most represented genera wereProtoperidinium(34 species),Ceratium(24 species),Dinophysis(15 species),Gonyaulax(11 species) andProrocentrum(8 species). A particular attention was given to potentially toxic species belonging to the generaDinophysis, AlexandriumandProrocentrum.The highest number of species (67 species) was recorded in July, and the lowest one (18 species) in February.