ABSTRACT The amount and distribution of radioactive material remaining on several atolls and incorporated into the flora and fauna of the Marshall Islands was determined one vear after their contamination by fallout from the March l, 1954 nuclear detonation of Operation Castle. Significant amounts of radioactive contamination were found in animals, food plants, water and soil samples. The highest concentrations of internally deposited activity were found in marine specimens taken from the northern Rongelap lagoon. Most of the activity in the marine specimens was contributed by Zr®-Nb?* and Rul%_Rn%_ No fractionation of Sr®9_Sr™ occurred in the tissue of the fish analyzed. Residual soil contamination was confined to the top several inches of soil, with movement indicated down to the lens water. The major radionuclide found in the tissues of land animals and plants was Cs}3?_ The island soil and lagoon water were contaminated principally by the rare earth elements, Ru!$ Rh% and Zr_Nb*”. The amount of activity in the specimens analyzed was generaily proportional to the external gamma reading in each of the areas. li