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inorganic and organic forms of Hg into the highly toxic methylmercury or dimethylmercury has made it clear that any form of Hg is highly hazardous to the environment (EPA 1980, 1985). The synthesis of methylmercury by bacteria from inorganic Hg compounds present in the water or in the sediments is the major source of this molecule in aquatic environments (Boudou and Ribeyre 1983). This process can occur under both aerobic and anaerobic conditions (Beijer and Jernelov 1979; Clarkson et al. 1984), but seems to favor anaerobic conditions (Olson and Cooper 1976; Callister and Winfrey 1986). Transformation of inorganic mercury to an organic form by bacteria alters its biochemical reactivity and hence its fate (Windom and Kendall 1979; Figure 1). Methylmercury is decomposed by bacteria in two phases. First, hydrolytic enzymes cleave the C-Hg bond, releasing the methyl group. Second, a reductase enzyme converts the ionic Hg to the elemental form, which is then free to diffuse from the aquatic environment into the vapor phase. These demethylating microbes appear to be widespread in the environment; they have been isolated from water, sediments, and soils and from the gastrointestinal tract of mammals--including humans (Clarkson et al. 1984). Methylmercury is produced by methylation of inorganic mercury present in both freshwater and saltwater sediments, and accumulates in aquatic food chains in which the top-level predators usually contain the highest concentrations (Clarkson and Marsh 1982). Organomercury compounds other than methylmercury decompose rapidly in the environment, and behave much like inorganic Hg compounds (Beijer and Jernelov 1979). In organisms near the top of the food chain, such as carnivorous fishes, almost all Hg accumulated is in the methylated form, primarily as a result of the consumption of prey containing methylmercury; methylation also occurs at the organism level by way of mucus, intestinal bacteria, and enzymatic processes, but these pathways are not as important as diet (Huckabee et al. 1979; Boudou and Ribeyre 1983). The biological cycle of Hg is delicately balanced, and small changes in input rates and the chemical form of Hg may result in increased methylation rates in sensitive systems (NAS 1978). For example, the acidification of natural bodies of freshwater is statistically associated with elevated concentrations of methylmercury in the edible tissues of predatory fishes (Clarkson et al. 1984). In chemically sensitive waterways, such as poorly buffered lakes, the combined effects of acid precipitation and increased emissions of Hg to the atmosphere (with subsequent deposition) pose a serious threat to the biota if optimal biomethylation conditions are met (NAS 1978). Mercury binds strongly with sulfhydryl groups, and has many potential target sites during embryogenesis; phenylmercury and methylmercury compounds are among the strongest known inhibitors of cell division (Birge et al. 1979). Organomercury compounds, especially methylmercury, cross placental barriers and can enter mammals by way of the respiratory tract, gastrointestinal tract, skin, or mucus membranes (Elhassani 1983). When compared with inorganic mercury compounds, organomercurials are more 10