Total and methyl mercury were analyzed in plankton and benthic inve11ehrates from Clear Lake, CA, an aquatic ecosystem contaminated from mining at the Sulphur Bank Mercury Mine over an 84-year period. Sediment total (primarily inorganic) mercury concentrations exceed 180,000 ng/g near the mine. Total mercury in Clear Lake biota (up to 855 ng/g in plankton, 41,671 ng/g in oligochaetes, and 27,686 ng/g in chironomids) was found to reflect the concentration of mercury in the organisms’ surroundings (water or sediment). Methyl mercury, however (up to 67 ng/g in plankton, 19.9 ng/g in oligochactes, and 61.9 ng/g in chironomids), typically was not correlated with inorganic mercury concentrations in water and250 sediment. Anomalously high concentrations of methyl mercury in biota at great distances from the point source of inorganic mercury suggests either (1) methyl mercury is produced in situ at sites with low inorganic mercury or (2) methyl mercury is produced in regions with high inorganic mercury and transported to other regions of Clear Lake by wind-driven currents. An increasing ratio of methyl to total mercury with increasing trophic level supports a bioaccumulation model for methyl mercury dynamics in Clear Lake biota and suggests that bioavailable mercury increases as a function of distance from the mine. Compared with other contaminated sites, Clear Lake’s plankton are relatively low in both total mercury and methyl mercury, despite the fact that sediments in this system are some of the most highly contaminated in the world. Benthic invertebrates at Clear Lake exhibit the highest total mercury values (by one order of magnitude higher than any other reported sites), yet have the lowest methyl mercury concentrations of any known contaminated sites.