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Levels of total mercury in the muscle (29 samples) and liver tissue (15 samples) of bluefin tuna (Thunnus thynnus) weighing 100-300 kg were determined by cold vapour atomic absorption (AA) spectroscopy. Tunas were previously captured in the waters of Malta, towed to the farm in the Adriatic Sea and fattened with defrosted herring and sardine for the period of 6 to 7 months. The purpose of the investigation was to determine the magnitude of mercury contamination and to ascertain whether the concentrations in muscle tissue exceeded the maximum level defined by the European Commission Decision (1 μ g/g wet weight). Total mercury concentrations in the muscle tissue of tunas ranged from 0.49 to 1.809 (median 0.899 μ g/g wet weight) while in the liver tissue it was from 0.324 to 3.248 (median 1.165 μ g/g wet weight). Total mercury concentrations in six samples of sardine ranged from 0.050 to 0.072 μ g/g wet weight while two samples of herring contained 0.020 and 0.053 μ g/g wet weight. Twelve out of 29 (41%) muscle samples of tuna contained mercury above the maximum level defined by the European Commission Decision. It is generally believed that mercury levels in Mediterranean fish are higher than those of the other seas or oceans due to numerous deposits of mercury ores and metallic mercury in surrounding countries.