Since the japanese mass poisoning in 1954's, classically named today "Minamata disease", mercury and its derivatives occupy a place of choice into the most worrying chemical pollutants for the human health and a lot of studies about the pathways by which mercury enters in its biogeochemical cycle or the environmental mercury levels are resumed in this paper. The most important aspects of mercury ecotoxicology are reviewed here: biological methylation reaction, bioaccumulation and biomagnification of these toxics in te aquatic ecosystems. The authors also describe the mercury and methylmercury metabolism and the symptomatology of the methylmercurial poisoning; with emphasis that infraclinic chronical manifestations, or large epidemiological interest, are not well known. It should be necessary to found a sensitive biochemical test, specific of this intoxication. The studies about the initial body burden assessment or the threshold values of mercury in easily obtainable specimens (blood hair) are only available for a statistical distribution. At individual levels, a number of factors occur, particularly the presence of selenium who should act as a biological antagonist of methylmercury.