- 66 - clams and scallops like zooplankton readily accumulate radionuclides in the form of particles but many of the radionuclides that occur in the digestive tract are not absorbed. In laboratory experiments, strontium radioiso- topes are taken up and deposited in the shells of oysters, clams and scallops but not in the soft tissues (Chipman 1959). In both the field and the laboratory, it was found that marine fish absorb few of the fission products from the digestive tract, radioisotopes in muscle, and do not concentrate strontium but do absorb zn©5 very rapidly. Although little csl37 nas peen found in the marine organ- isms from the Bikini-Eniwetok area, Csl37 has been taken up and concentrated in the muscle by fish and shellfish in laboratory experiments, and therefore is regarded as the fission product with the greatest potential hazard. The conditions that make for a higher concentration of csl57 in the laboratory than in the field are not known. In the field the concentration of cesium in four types of plankton is less than in sea water, as determined by Ketchum and Bowen (1958), which indicates that there is not a great demand for cesium at least by some types of plankton. Lastly, in both the field and laboratory observations it