-17- cesium and strontium, when introduced into the sea from fallout, would be present most likely in the insoluble or particulate state. otservations (9) Indirect evidence (8, 20) and support this view. direct The indirect evidence is based on geochemical studies in which the potential supplies of the elements to the seas from weathering of igneous rocks are compared with the solution in the sea. to fallout amounts of the elements present in These values cannot be applied directly analyses but they do provide the basis for an estimate of the fraction of a given fallout element that would remain in solution in the sea. The levels in the sea (ppm) of the naturally occurring forms of those elements reported in the present work are shown in Table 3. ring elements Also listed are the forms of the naturally occurin sea water, the percentage of the fallout ele- ments in solution according to direct and indirect evidence, the principal chemical state of the fallout in sea water, and the average percentages of the fallout elements observed in plankton samples collected about three months after pa DOEARCHIVES Of the radioactive fallout elements shown, only two, cesium and strontium, would occur normally in soluble form in the sea. The radioisotopes of these elements were found in the least amount in plankton. In contrast, the radioactive fission. products with the lowest solubilities, Zr95 and Cel44, were present in plankton in the greatest amount (20.2, 8.6 per cent) Ze