Seasonal variability of dissolved and particulate methylmercury (F-MeHg,P-MeHg) concentrations was studied in the waters of the Amazon River and its associated Curuai floodplain during hydrological year 2005-2006,to understand the MeHg exchanges between these aquatic systems.In the oxic white water lakes,with neutral pH,high F-MeHg and P-MeHg concentrations were measured during the rising water stage (0.70 ± 0.37 pmol/L,n =26) and flood peak (14.19 ± 9.32 pmol/g,n =7) respectively,when the Amazon River water discharge into the lakes was at its maximum.The lowest mean values were reported during the dry season (0.18 ± 0.07 pmol/L F-MeHg,n =10 and 1.35 ± 1.24 pmol/g P-MeHg,n =8),when water and suspended sediments were outflowing from the lakes into the River.In these lakes,the MeHg concentrations were associated to the aluminium and organic carbon/nitrogen changes.In the black water lakes,with acidic pH and reducing conditions,elevated MeHg concentrations were recorded (0.58 ± 0.32 pmol/L F-MeHg,n =16 and 19.82 ± 15.13 pmol/g P-MeHg,n =6),and correlated with the organic carbon and manganese concentrations.Elevated values of MeHg partition coefficient (4.87 ＜ Kd ＜ 5.08 log (L/kg) indicate that MeHg is mainly transported associated with the particulate phase.The P-MeHg enrichment detected in all lakes suggests autochthonous MeHg inputs from the sediments into the water column.The MeHg mass balance showed that the Cumai floodplain is not the source of P-MeHg for the Amazon River.