The aim of this study was to analyse fossil and subfossil diatom floras preserved in a sediment core taken from the Gulf of Gdansk, near the eastern end of the Hel Peninsula. The diatom record, supported by lithology and radiocarbon data, indicates environmental changes during the later stages of development of the Baltic Sea. In the Litorina Sea stage the diatom flora developed under marine conditions, whereas during the Post-Litorina stage there was a decrease in salinity. During the Subatlantic period the sublittoral diatom flora was replaced by shallow water diatoms originating from the coastal zone, and were redeposited in the deeper part of the basin. In the surface sediments the change to a presumably anthropogenically-influenced diatom flora can be attributed to the eutrophication of the Gulf of Gdansk.