-ll- In short, within the invertebrates and their symbionts alone complete biological cycles occur from land to sea and back again, from inorganic substances to organic and back again. The fishes of the waters in and about the Marshall Islands have received a major share of the attention in the study of the biological cycling of radioactive materials(Applied Fisheries Laboratory, 1950; Donaldson etal, 1956; Seymouret al. 1957 ; Welander, 1957 ; Lowman, Palumbo and South, 1957; Welander, 1958 ). Despite detailed study, the great variety of fishes with a correspondingly great variation in feeding habit make this a very difficult area in which to summarize results. In general, the fishes may be divided by feeding habit into three groups: the herbivores, omnivores and carnivores. Since the herbivores feed directly on the algae, the radioisotopes concentrated from the water by the algae are passed on directly to the fish, and from the fish to animal eating the fish. the The herbivores, represented by such fishes as the surgeonfish and parrotfish, have the greatest amount of radioactivity of the three major groups. Omnivorous fish such as the damselfish have less contamination than have the herbivorous fish, for they feed on more complex organisms. The herbivorous and omnivorous fish tend to concentrate the same isotopes found inthe plankton except for the radioisotopes which taken up only in trace amounts by these animals. are Zn§95 usually accounts for 50 per_cent or more of the total radioactivity in the organs of these fish and Fe99-959 comprises a major part of the remaining activity. The