Abstract: This review describes the occurrence of mercury pollution in aquatic environment.Much of the review discusses studies of mercury methylation and the opposite process-demethylation.By means of selection of mercury-resistant bacteria or fungi,it shows that equilibrium can be attained between the degradation and synthesis of methyl mercury in mineral mercuny polluted media,such as sediments.Effective water quality management of mercury contaminated natural waters necessitates quantitative models that can predict mercury transformations under various environmental factors.It emphasises that the accumulation of mercury and metyl mercury along food chain is most important polluted way for fish,poultry and human.The toxico logical effects of methyl mercury to fish and humen are also discussed.