ABSTRACT The general objectives were: (1) to measure the complex gammafield at a number of positions within 10,000 yards of each of the two underwater nuclear detonations (Wahoo and Umbrella), (2) to collect limited samples of airborne debris resulting from these detonations, and (3) to expose a numberof test panels to this same debris. The total gammafield was measured by means of about 20 gamma-intensity-time recorders installed on floating platforms located within a radius of 10,000 yards from surface zero. Samples of radioactive matertal deposited from the cloud were obtained by incremental collectors associated with the basic gamma-intensity-time recorders. Surface water activity was measured, and certain physicochemical parameters of the radioactive cloud were measured to calculate the free-field dose rates from the records obtained. Similar instruments supplemented by National Bureau of Standards film packs were used to determine gamma fields and total doses at various positions aboard three destroyers and a Liberty ship located within the area covered by the floating platforms. A comparison between shipboard fields and the local freefield is thus possible. During both Wahoo and Umbrella, nearly al! of the total gamma dose occurred within 25 minutes after zero time and was due to the passage of airborne radioactive material. The gamma-dose-rate records show pronounced and characteristic differences in the transtting gamma fields resulting from each of the two detonations. Gamma doses in excess of 100 r occurred within the first 15 minutes at downwind distances less than 16,000 feet from Wahoo and 14,000 feet from Umbrella. In both instances the residual field due to deposited radioactive material was relatively insignificant, although radioactive foam may represent a radiological hazard.