Plutonium is a man-made element whose behavior in the marine environment is inadequately known at present. It has been studied intensively in connection with production of weapons and power sources and has been characterized as an extremely toxic substance. Nevertheless, only a few dozen measurements have been made of concentrations in seawater and in the associated organisms and sediments. The first of these were as recent as 1964. There are reasons to believe its chemical behavior in the ocean is different from what has been observed on land, and that it will be difficult to predict how plutonium will distribute itself in the ocean. The consequences of increased environmental concentrations of Pu are discussed. (auth)