Methylmercury bioaccumulates in aquatic food chains and is able to cross the blood–brain barrier, making this organometallic compound a much more worrisome pollutant than inorganic mercury. We know that methylation of inorganic mercury is carried out by microbes in the anoxic layers of sediments and water columns, but the factors that control the extent of this methylation are poorly known. Mercury methylation is generally thought to be catalysed accidentally by some methylating enzyme^,, and it has been suggested that cellular mercury uptake results from passive diffusion of neutral mercury complexes. Here, we show that mercury methylation by the bacterium Geobacter sulfurreducens is greatly enhanced in the presence of low concentrations of the amino acid cysteine. The formation of a mercury–cysteine complex promotes both the uptake of inorganic mercury by the bacteria and the enzymatic formation of methylmercury, which is subsequently released to the external medium. Our results suggest that mercury uptake and methylation by microbes are controlled more tightly by biological mechanisms than previously thought, and that the formation of specific mercury complexes in anoxic waters modulates the efficiency of the microbial methylation of mercury.