Waterbirds are exposed to many contaminants. Heavy metals such as lead and cadmium are known as the most important toxic pollutants in an aquatic ecosystem. Poisonous elements had never been reported in larus birds (Larus heuglini) of the Iranian Mangrove Biosphere Reserve. The aim of this investigation was to measure levels of metal concentration in liver, kidney and muscle tissues of larus in the Mangrove Biosphere Reserve. To this end, fifteen birds were collected on November 2010. The results indicated that the average lead, cadmium and zinc levels were highest in kidney and lowest in muscle. There was no significant difference in tissue concentrations of Cd and Pb between males and females. In the present study, levels of Cd and Pb in tissues showed no change with age. However, the concentrations of trace elements in Larus heuglini were generally comparable to values reported in other studies. The results of the investigation do not show an excessive exposure to cadmium, whereas some interesting data have emerged in the case of lead. The concentration of the latter in the liver and the kidney of larus seem to suggest the possibility of recent exposure. Kewords: Metals, Mangrove Biosphere, Ecosystem, Larus heuglini.