In order to evaluate aquatic environmental mercury (Hg) and methylmercury (MeHg) contamination, a wide variety of fish species were sampled from some tributaries of the Brazilian Amazon river system, the Balbina reservoir and the Pantanal watersheds. These water bodies present different mercury inputs and biogeochemical characteristics. Amazon fish, which are the main pathway of MeHg to the local population, are the most important protein source for them, and fishing is a significant economic activity throughout these regions. MeHg in fish samples (164) were analysed with an efficient extraction technique and measured by GC-ECD. Analytical quality was checked through intercomparisons between two laboratories with local samples and a certified standard from IAEA. MeHg concentrations of carnivorous, omnivorous, detritivorous and herbivorous species ranged from 0.1 to 1.25 mg kg À1 wet wt and the mean percentages of MeHg to total mercury were usually higher than 80%. Carnivorous (piscivorous) fish, which represented 74% of all samples, effectively showed higher MeHg concentrations as well as a higher MeHg/total mercury ratio in muscle tissue than fish from lower trophic levels. In general, MeHg concentrations in carnivorous fish were higher in places close to goldmining activities, the Madeira river and the Tapajo ´s river near Itaituba city. The MeHg/total mercury ratios in fish were higher in non-impacted areas and with smaller amounts of suspended particulate materials (Negro river, Balbina reservoir and Pantanal watershed). No MeHg seasonal variability was observed in Serrasalmus rhombeus (carnivorous fish) from the Madeira river basin during the year. High variability in MeHg levels was observed in muscle of the same carnivorous species fish with similar weights sampled at the same place and by similar nets.