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Plutonium distribution was studied in an undisturbed sediment core sampled from the Tvären bay in the vicinity of the Studsvik nuclear facility in Sweden. The complete analysis, including minor isotopes, of the Pu isotope composition ( 238 Pu, 239 Pu, 240 Pu, 241 Pu, 242 Pu, and 244 Pu) allowed us to establish the Pu origin in this area of the Baltic Sea and to reconstruct the Studsvik aquatic release history. The results show highly enriched 239 Pu, probably originating from the Swedish nuclear program in the 1960s and 1970s and the handling of high burn-up nuclear fuel in the later years. In addition, the 244 Pu/ 239 Pu atomic ratio for the global fallout period between 1958 and 1965 is suggested to be (7.94 ± 0.31)·10 -5 . In the bottom layer of the sediment, dated 1953-1957, we detected a higher average 244 Pu/ 239 Pu ratio of (1.51 ± 0.11)·10 -4 , indicating the possible impact of the first US thermonuclear tests (1952-1958).