The plankton collected at 15 stations in Ariake-and Yatsushiro-kai were divided into phytoplankton (diatoms), zooplankton (mainly copepods) and large sized thaliacean, and their mercury contents determined. The mercury concentrations in the plankton were high at several stations in the vicinity of Minamata and decreased with increasing distance from Minamata. On the other hand, the mercury concentrations in the plankton were of the same order as those in the fishes and shellfishes collected in the same areas. From these results, it is considered that the degree of pollution by mercury in the surveyed region was rather low except in the vicinity of Minamata where mercury had been formerly discharged in large quantities, and that the absorption of mercury in the fishes and shellfishes shows little dependence on the food chain.