The concentration of mercury in selected organs (muscles, liver and gills) of five fish species was measured by flameless cold vapor atomic absorption spectrometry (CV AAS). The fish species examined: freshwater (pike, Esox lucius (L.), roach, Rutilus rutilus (L.) and bream, Abramis brama (L.)) and marine (mackerel, Scomber scombrus and flounder, Platichthys flesus), were bought from supermarkets of Olsztyn (north-eastern Poland) over the period from November to December 2006. Differences in the total mercury content were found both between species and organs. The concentration of mercury turned out to be higher in muscles of freshwater fish than in other organs (p≤0.05). Muscles of flounder had also more mercury than gills and liver, but differences were statistically significant only between muscles and gills (p≤0.05). The content of mercury in the liver of mackerel was higher than in gills (p≤0.05) and muscles, although did not differ statistically (p>0.05). The higher levels of mercury (0.226 mg/kg and 0.084 mg/kg, respectively) were determined in muscles (p≤0.05) and liver (p≤0.05) of pike as compared to the other fish studied (with the exception of mackerel), because the concentration of Hg in liver of these fish was not statistically significant (p>0.05). A significantly higher mercury content (0.034 mg/kg) was recorded in gills of mackerel than in those of bream (p≤0.05). The muscle tissue of freshwater fish contained a higher concentration of mercury (0.135 mg/kg) than that of marine fish (0.052 mg/kg), (p≤0.05). In turn, the content of mercury (0.033 mg/kg) in gills of marine fish was higher than in the freshwater fish (0.019 mg/kg), (p≤0.05), whereas the content of mercury in liver of freshwater fish was close to that of the marine fish (0.051 mg/kg and 0.053 mg/kg, respectively) (p>0.05).