15 in 1954 to a time 700 days later is given in Table 3. That dif- ferent tissues have different rates of decline is evidenced by the shift in relative positions of the samples in regard to radioactivity between the first and last collections. Since the beginning of the series of weapons tests at Bikini and Eniwetok a program to determine the biological and geographical distribution of fallout in the vicinity of the test site has been a part of each test operation. The biological program of the Laboratory of Radiation Biology, University of Washington (formerly the Applied Fisheries Laboratory), since the the first operation at Bikini in 1946 has recently been reviewed by Donaldson (1959a). Other reports on radioactivity in the oceans and of biological programs associated with weapons testing programs are included in recent bibliographies by Davis (1958) and by Wallace (1958). The fallout at Rongelap Atoll has received special attention. This is the one area in the world where fallout has been great enough to require that the population be evacuated. Although the level of radioactivity is now low and the natives can live safely at Rongelap, the area provides a wonderful opportunity for a study of radioisotopes in a natural environment. The history of the Rongelap studies follows. Since the radioactive contamination of Rongelap Atoll on March 1, 1954, repeated studies have been made of the extent and distribution of the contamination. Eleven collections were made