The development of the blue-green algae in the Gulf of Gdansk begins in May and lasts till October. To the stable compo-nents of the filamentous forms belong Aphanizomenon-flos aquae, Nodularia spumigena, Planktolynbya limnetica, Pseudanabaena limnetica and Planktothrix sp. The coccoid forms are represented by Snowella septentrionalis, S. lacustris, Merismopedia punctata, M. tenuissima, Woronichinia compacta, Rabdogloea linearis, Cyanodictyon imper,fectum, C.planctonicum. In the years 1992 and 1993 the openwater area of the Gulf of Gdansk was characterised by the summer (July) domination of two species of blue-green algae: Microcystis aeruginosa and Aphanizomenon flos-aquae. The presence of Nodularia spumigena was also noticed. In 1994, the extremely dense bloom of Nodularia spumigena occurred with the biomass increase to 130 mg?C? m^3. The intensity of this bloom could be linked to temperature which in summer 1994 was very high and reached over 22?C. In the years 1996 and 1997, temperature did not attained so high values and the intensity of blooms was lower.