[en] The research history and up-to-date research of plutonium in the oceans, performed at Nakaminato Laboratory for Marine Radioecology, National Institute of Radiological Sciences, are described; plutonium in the submarine sediments, the marine organisms, and the sedimentary particulates. Next, the origin and the behavior of plutonium in the marine environment have been investigated by using the difference of ²⁴⁰Pu/²³⁹Pu value. The application of the differences of this isotope ratio to the research of dynamic marine movement is described. The future research plans are the preparation of mapping of this isotope ratio over the world in the sea, the measurement of vertical distribution of the isotope ratio varied with time, and the elucidation of the scavenging process by the measurements of plutonium concentration and isotope ratio in the sedimentary particulates. (M.H.)