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The radioisotopes of caesium (¹³⁷Cs) and strontium (⁹⁰Sr) make the greatestcontribution to the radioactivity level due to artificial radionuclides in the BalticSea, where the level of ¹³⁷Cs contamination is higher than in any otherpart of the world ocean. The main sources of man-made radionuclides are the Chernobylaccident in 1986 and the nuclear weapons tests carried out in the 1950s and 1960s.This study discusses the distribution patterns and trends in activity concentrationsof ¹³⁷Cs and ⁹⁰Sr recorded in various compartments of the marineenvironment of the southern Baltic Sea. It is based on an investigation of radioactivesubstances as part of the Polish National Environmental Monitoring Programme.In 2010 the average concentration of ¹³⁷Cs in the southern Baltic was35 Bq m^-3, while the level of ⁹⁰Sr in these waters has remained at much the same levelin recent years (ca 8 Bq m^-3). The distribution of isotopes in the bottomsediments reflect historical events that can be identified in sedimentprofiles. The activity concentrations of the caesium isotope are the highestin sediments from the Gulf of Gdansk, whereas the least polluted sedimentsare found in the Bornholm Basin, in the western part of the southern Baltic.The highest concentrations of ¹³⁷Cs in benthic plants were measured in the red alga Polysiphonia fucoides: 22.3 Bq kg^-1 d.w. in June and 40.4 Bq kg^-1 in September. These levels were much higher than those found in the bivalve Mytilus trossulus (7.3 Bq kg^-1 d.w.). ¹³⁷Cs concentrations in fish have decreased in time, reflecting the trends recordedin seawater. In 2010 the respective ¹³⁷Cs activities in Clupea harengus, Platichthys flesus and Gadus morhua were 4.7, 4.9 and 6.6 Bq kg^-1 w.w