Since the late 1980s, several studies have shown that human populations in the Amazon basin are exposed to high mercury levels in their fish diet. Gold mining, which releases the metal during the amalgamation process and erodes soils naturally rich in mercury, is regarded as the main contamination source. Here, we present the results of a comparative study of mercury distribution in the water and fish of two adjacent rivers in French Guiana, with and without gold mining activities. As a consequence of a marked difference in suspended particulate matter between the two systems, total mercury concentrations in unfiltered water samples were higher in the mined river (25.4-34.9 ng L(-1)) as compared to the reference one (2.1-5.4 ng L(-1)). Surprisingly, no significant differences were observed in mercury concentrations between 13 common fish species at upstream sites. In sharp contrast, mercury concentration of fish caught downstream a hydroelectric reservoir, where the two rivers flow, was up to 8-fold higher than that upstream. Mercury speciation measurements allowed one to relate these differences in fish to the water distribution of monomethylmercury, the mercury chemical species that biomagnifies along aquatic foodwebs. Indeed, mean dissolved monomethylmercury concentrations were low and similar in both rivers (0.03-0.06 ng L(-1)), while they were 10 times higher (up to 0.56 ng L(-1)) in the water outflowing the hydroelectric dam. Dissolved monomethylmercury determinations along a water column profile suggest that methylation of inorganic mercury occurs in the deep anoxic part in reservoir. We conclude that mercury mobilization related to gold mining is not solely sufficient to account for high concentrations in fish and that environmental conditions that favor mercury methylation, such as anoxia, are needed.