The work aimed at determining in which predatory fish tissue mercury is cumulating to the highest degree. Mercury content was investigated in particular tissues and organs of pike. Fishes bought at random in Warsaw fish wholesale company in November 2005 comprised the experimental material. Before sampling fishes were weighed and measured. The samples of the following tissues were collected for the investigations: scale, fins, liver, spleen, swim bladder, heart, frontal and dorsal kidney, spawn or milt, eyes, gills, skin, bones, brain and intestine. Mercury content in the collected samples was determined using the analyzer of mercury traces controlled by the computer programme AMA 254. A correlation between the level of mercury concentration and the age and body weight of fish was observed as a result of the performed investigations. The tissue which cumulates mercury to the highest degree is the muscle tissue, significant amounts of this element were also noted in the intestine and liver which proves that the main way of mercury entering the fish organism is the alimentary tract.