The toxicological consequences of Hg releases to the environment are largely governed by the conversion ofinorganic Hg to the most toxic methylmercury (MeHg), that is biomagnified through aquatic food chains. To gain further insight on the biological and physico-chemical factors controlling MeHg production and distribution among freshwater sediments and water, we used a sensitive and specific radiochemical procedure, developed at the National Institute for Minamata Disease. Systems containing 203Hg2+-spiked sediment cores (0.7 g total Hg g-1 d.w.) and overlying water, both from a pristine mountain stream in Southern Japan, were incubated for 21–38 days in different conditions. Inorganic Hg and MeHg in sediment and water were extracted in dithizone-benzene and measured after separation by thin-layer chromatography. The conversion of added Hg to MeHg was 3.0 to 13.7% in sediments, with a tendency for higher proportions in the top layers. Surprisingly, more MeHg was found in the sediment (11.3%) and water (66.5%) of a system bubbled with air than in one bubbled with nitrogen (4.2 and 44.1%). Artificially increased levels ofbioturbation reduced by half the MeHg concentrationsand % of added total Hg in sediment and water. In allsystems, 55–68% of total Hg and MeHg in water wereassociated to suspended particles >1m. MeHgbioaccumulation factors (BFs) from water ranged270–8100 and from sediment, 0.2–5.7 (wet weight basis). BFs in relation to water where 3 times higher for MeHg than for total Hg.