Discrete water samples for the determination of phytoplankton abundance and community composition were collected with a monthly frequency at 0.5, 5, 10 and 15 meters in the Gulf of Trieste – time series station (station C1). The site is part of the LTER-North Adriatic site. Samples were fixed with Ca(HCO3)2-buffered formaldehyde (0.8% final concentration) and cell abundance and species composition of phytoplankton were estimated according to Utermöhl’s method (1958), using an inverted microscope equipped with phase contrast, at 320-400x final magnifications. The overall aim of Long Term Ecosystem Research in the coastal area of the Gulf of Trieste, in the northernmost part of the Adriatic Sea and of the Mediterranean basin is to gain a better understanding of the temporal dynamics of phytoplankton bloom evolution, with particular focus on the response to natural and man – induced environmental changes. General information can be found at: http://nettuno.ogs.trieste.it/ilter/BIO/index.html