Depth variations of total mercury (Hg) and methylmercury (MeHg) concentrations were obtained in cores from nonvegetated sediments, sediments colonized by Sarcocornia fruticosa, Halimione portulacoides, and Spartina maritima and below-ground biomass in three Portuguese estuaries. Similar analyses were also performed on the above-ground plant tissues. Concentrations in below-ground biomass exceeded up to 9 (Hg) and 44 (MeHg) times the levels in sediments. Mercury and MeHg in below-ground biomass were up to 400 (Hg) and 4700 (MeHg) times higher than those found in above-ground parts, indicating a weak upward translocation. Methylmercury in colonized sediments reached 18% of the total Hg, which was 70 times above the maximum values found in nonvegetated sediments. Concentrations of MeHg in vegetated sediments were not related to plant type but were linearly proportional to the total mercury levels. The analysis of below-ground biomass at high depth resolution (2 cm) provided evidence that Hg and MeHg were elevated. The higher enrichment factors were found where the shifting of redox conditions suggested high microbial activity. Mercury and MeHg in below-ground tissues were a function of total levels in sediments and again were not plant-specific. These results suggest that the bioremediation of mercury-contaminated sediments is likely to increase the formation of methylmercury.