-26- approach each other in pattern with passage of time. the early period after the shot there is, of course, variety of radioactive isotopes present than later, During a greater and there is some indication that selective uptake might be more marked or selective exclusion less marked during this period. As the shorter-lived isotopes decay and decrease in importance, leaving fewer radioactive materials available, the decline and decay of radioactivity in the different tissues and species show greater similarities. The data presented do not permit exact distinctions between tissues or between different species on the basis of differences in the uptake of radioactive materials. It appears from the decay curves and the decline trends that the fish tissues differ mainly in orders of magnitude rather than in quality after the first 100 days, although there is the possi- bility of different isotopes with similar half lives being present. Radiochemical analysis done within two months on fish collected two to three months after shot contained MnD4 , Fe-?,Coo7, C058, C090, and gn©5, with Fe°> and zn©5 as the dominant isotopes. Fish collected within one month after the 1954 shots and analyzed January 1957 contained 95 percent Fe; Mn54 Col’, and 669 (Lowman, Palumbo and South, 1957) contributed the remainder of the radioactivity. There may be fission products in the fish the first few weeks or months after the shot, but after four or five months fission products contribute very little, the total radioactivity in the fish. if any, to