Mercury (Hg) is a natural and widespread trace metal occurring in the environment but, at high concentrations in environment, it is considered as a priority hazardous substance because it damages the central nervous system in severely exposed humans. The case of Hg is quite particular because its bioamplification is different for its different chemical forms. Monoethylmercury (methylmercury) is more easily biomagnified and bioconcentrated than inorganic Hg. The consumption of Hgcontaminated fish is the most common exposure pathway of methylmercury for humans. In aquatic systems, Hg methylation occurs mainly in sediments. Mercury methylation depends on many parameters such as concentrations of sulphide, iron, mercury, organic matter, and, in natural environments, seems to be the result of sulphate- and iron-reducing bacteria activities (SRB and IRB, respectively). The studies performed to explain methylation investigating only one strain and/or bacteria group revealed that some microorganismgroups were able to methylate Hg as a detoxification process. In general, the studies were based on the cultures, isolation and identification of the strains. Other studies added obligate compounds for one selected metabolic pathway and then the Hg transformation was measured. Finally, added obligate compounds were measured for one selected metabolic pathway (i.e molybdate to inhibit sulphate-reduction) and the bacteria involved in methylation were determined.