Dissolved and particulate activities of caesium-137 are presented for the coastal and open southern Baltic Sea waters, fluvial waters (the Vistula and Swina rivers) and lagoon waters (the Vistula Lagoon) during 1996-1999 and compared to those collected before the Chernobyl accident. ¹³⁷Cs was measured in the Baltic Sea waters at different depths as well as 0.2-0.3 m above the sea bed. In a layer of the Gdansk Bay waters (the Gdansk Deep location), extending from the surface down to 1-2 meters above the sea bed, caesium-137 concentrations ranged between 57 and 66 Bq m^-3 in 1999, whereas in the layer situated 0.2-0.3 m above the sea bed its concentration increased up to 87 Bq m^-3. This phenomenon was also observed in 1994 and 1998. The concentrations of caesium-137 in riverine suspended particulate matter (r SPM) are lower than in the marine SMP (m SMP) and its activity attains respectively 36 and 222 Bq kg^-1 based on dry weight. The proportion of caesium-137 in the suspended form in the Baltic Sea waters does not exceed 2 per cent, while in the fluvial suspended form it is as high as 55 per cent.