higher the level the lower the total amountof radioelements contained therein. Reproductive rates, average life spans, and average size of the organisms comprising each trophic level modify the total accumulation of radioactive material in that level. The initial stage in the uptake of radioactive elements by nannoplankton probably occurs by adsorption of the material on to the surface of the organism by means of ionic or covalent linkages. In plankton collected within 48 hours after contamination, the principal isotopes were Np29°, Mo®—Tc%m, Te132_[182, 7237, Ruy 103, 105, 106__Rf,103. 105, 106, Cel4i, 144__Pyl41, 144, Bgl40__Tal40 and Zr95—Nb%, At one week after contamin- ation the principal isotopes in descending order of abundance were Co*®, Bal4°__Lal40, Fe55, 58 Co’, Zr®5__Nb?*5, Ru!6s, 105, 106__Ry103, 105, 106 Co 89, Zn%, Np2%, and U?37_ At six weeks Zn®5, Fe55, Zr®5—Nb®5, Co®8, Co47, Cel44—Pr!44, Ru?6_Rh66, and Co® were present in the plankton in the above order of occurrence. The initial uptake of radioelements by plankton included, almost entirely, radioactive anions. Within a week, however, radioactive transition elements of the first series (cations) accounted for the major part of the radionuclides in the plankton. W185, which has no known metabolic function, was not retained by plankton, although the initial contamination of these organisms by radiotungsten was high. The second stage of accumulation of radioactive contamination by marine plankton occurs throughthefilter-feeders, which probably ingest both inorganic and organic particles. In the contaminated area near the Eniwetok Test Site, only a small fraction of the total radioactivity in the sea was associated with the plankton. In four surveys taken up to 8 weeks following contamination, the average total activity in the water was 40,000 times that in the contained micro-macroplankton. However, the radioactivity per unit volume of plankton was much higher than that for an equal volume of water. In the fish samples collected in the lagoons at Eniwetok and Ailinginae Atolls, Fe55, Zn®5, Co47, 58, 60, Mn54 and Zr98—Nb® accounted for the major part of the radioactivity. In tuna fish samples taken in the open sea near the test site, cations contributed 100% of the activity. Zn®> was highest and Fe55 was second in abundance. Co4’: 58, 6° and Mn54 were present in low amount, and the low level of the cobalt isotopes suggests a rapid turnover rate for this element in marine fishes. The amount of radioactive cobalt in plankton increases, with respect to that of Zn®5 and Fe55, with time after contamination. This may be related to a rapid turnover rate for cobalt and a low rate for zinc and iron in pelagic fishes which feed indirectly upon the plankton. The uptake of radioisotopes by symbiotic organisms was studied in two related genera of tropical clams, both of which contain symbiotic algae. One clam is more closely associated with the algae than the other, but both obtain a major part of their food through their symbionts. Waste products from digested algae are stored in the kidneys of the clams, mostly as concretions. Of the radioisotopes deposited in these organs, 95°%are cations. However, in the visceral mass 94°% of the contained radioisotopes are anions. The amount of Zn® deposited in the kidneys of the clamsis inversely related to the closeness of the symbiotic association between the clam and the algae. 136