Zooplankton samples were collected from nine stations in the three inland sea regions (Minamata Bay, Yatsushiro-Kai, and Ariake-Kal) along the western coast of Japan Between September 1980 to December 1986 and were analysed for total mercury. Some samples were also analysed for methyl mercury, copper, zinc, iron, manganese, cadmium, lead and nickel. The sediment in Minamata Bay still contained high mercury concentrations. The mercury levels in zooplankton suggested that the Minamata Bay where mercury was discharged about 30 years ago is still contaminated. Concentrations of total Hg in zooplankton decreased with increasing distance from the point of incident. Total Hg varied in the range of 0.037 to 5.40 mu g g/1 (dry) whereas methyl mercury content was 0.01 to 0.25 mu g g/1 (dry). Among the various groups of zooplankton studied chaetognaths, bivalve, and crustaceans showed higher rate of bioaccumulation. The distribution of other heavy metals was not abnormal and the values were below the widely accepted limits