Methylmercury (MeHg) production from inorganic mercury in natural environment leads to bioaccumulation in fish, putting human being under the risk of exposure. This study investigated the transformation of enriched stable isotope tracer, 199Hg2+, in sediment slurry of the Florida Everglades. Incubation experiments were conducted under four different laboratory conditions. The results suggest that methylation of mercury mainly happened under anaerobic condition, with microbial activity playing the major role. The relative methylation rate was determined to be 1% per day at the first few days, and then this transformation process slowed down. At the end of incubation experiment, totally around 20% of the isotope tracer 199Hg2+ was transformed to its methylated form, Me199Hg. This high potential of mercury methylation partially accounts for the contamination and bioaccumulation of MeHg in the relatively pristine Florida Everglades ecosystem.