The permanent growing of the phytoplankton biomass of the Baltic Sea during the last 10 years was accompanied by the changes in characters of seasonal vegetation. These changes were expressed also in the structure of phytoplankton community. In eutrophic waters of the Gulf of Finland from May till July 1995 the phytoplankton biomass reached 10 g m^-3 (May). This level was near that of maximum biomass in 1984 (HELCOM, 1996). From May to July biomass decreased in succession on one order: 10500-1030-150 mg m^-3. In May diatoms dominated in abundance (68%) and dinoflagellates in biomass (86%). In June diatoms dominated both in abundance and in biomass (82 and 63% accordingly), in July cyanobacteria were mostly abundant (75 and 44% accordingly). There are 8 potentially harmful and toxic species of cyanobacteria and 5 of dinoflagellates, which are typical in the Gulf of Finland. In the central Baltic (Gotland region) inter-annual trend of biomass was not expressed. In the period from May to July 1995 the biomass alters: 930-138-70 mg m^-3. In May dinoflagellates dominated both in abundance and in biomass (68-58%), in June - cyanobacteria (87-56%). The content of Ebriidea (Ebria tripartita) was 5-22%. In July dinoflagellates dominated. In the South-West Baltic in summer (June, 1998 and July, 1996) phytoplankton biomass was low (96 and 72 mg m^-3). In June diatoms and cyanobacteria dominated in abundance (35 and 33%) and Ebriidea in biomass (49%). In July Ebriidea dominated both in abundance and in biomass (45-61%). In autumn (October, 1997) diatoms developed as usual (1.2 g m^-3).