ABSTRACT Project 2.7a was an outgrowth of Project 2.7, the genesis of which is deacribed in the Project 2.7 report, Reference 1. During the 2.7 surveys, samples of marine organisms of the deep sea were collected by Scripps Institution of Oceanography (SIO) and were later analyzed by SIO and the U.S. Naval Radiological Defense Laboratory (NRDL). It was the objective of Castle 2.7a to ascertain and to report on the general relationship pertinent to the uptake of fission products by the marine organisms collected during the 2.7 survey in order to form a background for more extensive tests on Operation Wigwam. Gross beta activities, beta absorption curves and gamma spectra were analyzed, after identification of the organisms. A radiochemical analvsis was performed by NRDL. it was found: (1) that marine organisms concentrate activity from fallout fission products in the water by factors of the order of 1,000, (2) that the partition of fallout fiasion products in the ocean is profoundly influenced by biological processes and that a purely physical model is inadequate to predict distribution, (3) that the feeding mechanism of the organism does not clearly determine the amount of activity assimilated, (4) that ihcre ig evidence of fractionation of isotopes by different organisms, and (5) that there is some evidence that fincly dispersed activity is retained more or less proportionally with tne dry weight of the organism.