Mercury methylation and partitioning between sediment, water and fish were studied in river sediment spiked with mercuric chloride at levels of 1, 5 and 10 ppm as Hg. Maximum methylmercury production in the sediment occurred during the first week of incubation, with concentrations reaching 45.5 ngrg dry wt. . Equilibrium conditions were Ž . established 3]4 weeks after the contamination of the sediment with mercury. Methylmercury partitioning between sediment and water yielded methylmercury levels of 25]154 ngrl in the fish aquariums and 0.26 ngrl in the fish-free control after 1 week of incubation. Equilibrium levels were 1.5]5.5 ngrl and 0.53 ngrl, respectively, during the 7th week. Over 50% of the methylmercury in whole water were in soluble form or associated with colloidal particulate -1 mm. Mercury concentration in fish increased almost exponentially from 30 ppb to an average of 345 ppb within 3 weeks. Mercury uptake rates by fish were in the range of 10]18 ngrg per day during the 2nd and 3rd weeks, high rates occurring in water with methylmercury to total mercury ratios )0.45. The partition coefficients for total mercury and methylmercury between fish and water Ž . K were 5000]7000 and 10 000]22 000, respectively. Large f ] w differences were observed in methylmercury production in sediment]water incubation with fish and the fish-free control. Equilibrium methylmercury concentrations in sediment were in the range 15]32 ngrg in the aquariums containing fish and 3]4.5 ngrg in the fish-free control. The significance of fish in mercury methylation in the aquariums still remains to be clarified as fish itself cannot methylate mercury in vivo.