ABSTRACT The bomb debris from surface, land and water shots at Operation CASTLE was studied to determine the physical, chemical, and radio-~ chemical characteristics, The fallout from the surface land shots consisted chiefly of irregular white particles 25 » to 2 mm in diameter. They were derived from coral and had the redioactivity concentrated near their surfaces, About 5 per cent of the activity in the solid fallout was water soluble; 95 per cent dissolved in dilute acetic acid. The fallout from the surface water shots was invisible both in the air and after it had deposited, It was collected on special filters and on a film by electroatatic precipitation, The filters and film and their autoradiographs were studied microscopically. These studies showed that the fallout consisted of microscopic solid crystals and small droplets, The autoradiographs indicated the presence on the filters of many particles which were invisible under the microscope, The major part of the radioactivity was associated with crystalline aggregates and droplets up to about 2 mm in diameter. Water dissolved from 60 to 90 per cent of the radionuclides from this type of fallout, Fallout and cloud samples from land and water shots were analyzed chemically for major constituents and trace elements including many of the radionuclides. Coral and sea water contributed the major constituents, bomb products being present in trace concentrations, Radicchemical analysis showed the valley of the fission product yield curve was about 20 times higher and the heavy wing at mass 156 about 6 times higher than the yield curve from thermal neutrons on 0235, The important induced radionuclides were 0239-Np239, 0237, and 0240, The presence of these had a marked effect on the decay curves and energy spectra especially at intermediate times after detonation. The neptunium was distributed between oxidation states; iodine occurred princivally as iodide, The information obtained from these studies has aided in (a) an understanding of the mechanism of formation of the fallout, (b) assess- ing the radiological situation in fallout areas, (c) synthesizing simu- lants for laboratory studies, and (d) interpreting data obtained in proof tests of countermeasures for ships.