The muscle tissue samples of smelt (Osmerus epelanus), herring (Clupea harengus), sabrefish (Pelecus cultratus), roach (Rutilus rutilus) , bream (Abramis brama), ruff (Acerina cernua), perch (Perca fluviatilis), pikeperch (Stizostedion lucioperca), flounder (Platichthys flesus), brown trout (Salmo trutta) and burbot (Lota lota) collected in the Firth of Vistula, the Lower Vistula River at the B3otnik site and the Gulf of Gdansk in May and June 1997 have been analysed for the total mercury content. The method of mercury measurement was flameless cold-vapour atomic absorption spectroscopy (CV-AAS) after wet digestion of the samples with concentrated nitric acid in a whole glass system. The study did indicated around twice higher concentrations of Hg in fish collected from the Lower Vistula River than from the Firth of Vistula. Total mercury concentrations for a relatively large number of the specimens (between 12 and 41) of bream, perch, ruff, herring, flounder and pikeperch caught in the Firth of Vistula did correlated (0.05 < p < 0.01) with a total body length and body weight (age) of fish. Any of the fish species examined did contained mercury in concentration exceeding 300 ng/g wet weight. Nevertheless, smelt, herring, sabrefish and ruff the were the only specimens mature, while other fish examined were rather of small body size. Ongoing study on mercury in the ecosystem of the Lower Vistula River will further focus on a larger number of fish species and specimens and including also the individuals large in body size.