Methylmercury (MeHg) degradation was investigated along an eutrophication gradient in the Florida Everglades by quantifying ¹⁴CH₄ and ¹⁴CO₂ production after incubation of anaerobic sediments with [¹⁴C]MeHg. Degradation rate constants (k) were consistently ≤0.1 d^-1 and decreased with sediment depth. Higher k values were observed when shorter incubation times and lower MeHg amendment levels were used, and k increased 2-fold as in-situ MeHg concentrations were approached. The average floc layer k was 0.046 ± 0.023 d^-1 (n = 17) for 1−2 day incubations. In-situ degradation rates were estimated to be 0.02−0.5 ng of MeHg (g of dry sediment)^-1 d^-1, increasing from eutrophied to pristine areas. Nitrate-respiring bacteria did not demethylate MeHg, and NO₃^- addition partially inhibited degradation in some cases. MeHg degradation rates were not affected by PO₄^3- addition. ¹⁴CO₂ production in all samples indicated that oxidative demethylation (OD) was an important degradation mechanism. OD occurred over 5 orders of magnitude of applied MeHg concentration, with lowest limits [1−18 ng of MeHg (g of dry sediment)^-1] in the range of in-situ MeHg levels. Sulfate reducers and methanogens were the primary agents of anaerobic OD, although it is suggested that methanogens dominate degradation at in-situ MeHg concentrations. Specific pathways of OD by these two microbial groups are proposed.