A hydrobiogeochemical investigation of a small headwater peatland located in the Experimental Lakes Area, northwestern Ontario, Canada, examined the surface and subsurface hydrologic pathways and their relation to the movement and spatial variability of methylmercury (CH3Hg+). The hydrology of the peatland controls the mass flux of CH3Hg+ to a downstream pond from the terrestrial ecosystems, and influences the production and/or accumulation of CH3Hg+ in the peatland. Distinct zones of groundwater recharge and discharge were observed within the peatland, and these corresponded, to low and high CH3Hg+ concentrations, respectively, in pore water. The magnitude and flux of CH3Hg+ from the peatland was governed by the area of the peatland and surrounding uplands contributing runoff. There was a threefold increase in CH3Hg+ concentration as stream base flow passed from the origin of the stream at the hillslope-peatland margin to the stream outlet at the peatland-lake interface. This relative increase in concentration was maintained during storm flow conditions even though the discharge was up to 10 times greater, indicating that the peatland is a large source of CH3Hg+. Methylmercury-laden peat pore water found near the surface of the peatland in areas of groundwater discharge moves to the stream as the water table rises to the peat surface. Of a total of 12.6 mg CH3Hg+ leaving the peatland during the 112-day study period, less than 1% was transported directly by groundwater, 41% was transported in stream base flow, and the remaining 58% was transported by storm flow. However, storm flow conditions occurred only 16% of the time, indicating the significance of episodic CH3Hg+ flux from headwater catchments on the Precambrian Shield. ¿ American Geophysical Union 1996