ABSTRACT The objective of this project was to documant the characteris= tics of the close-in radioactive fallout resulting from the surface land or water detonations of high yield nuclear devices in order to provide information for the evaluation of:(1) the immediate hazards associated with the residual contamination from such bursts,(2) the mechanism of particle formation and distribution,and (3) the character~ istics and significance of the radioactive debris distributed by base surge phenomena provided that a significant base surge ‘s caused by surface water bursts. The objective was accomplished by sampling the fallout with intermittent fallout collectors and by analyzing the particulate and liquid matter for activity, decay, energies, and particle size distribution. Fallout stations were set up in varying arrangements for Shots l; 2; 3, hy and be When significant fallout occurred at an island after any of these shots, it apparently began to arrive there within six minutos after the detonation. The maximum activity per sampling time interval resulting from Shot 1 and other shots having yielde of the same order of magnitude arrived at all sampling stations during the first hour after the detonation. Extrapolation of the beta activity had indicated rates as high as 1.3xL014 dpm/ft~ 1 to 6 min after the detonation. Most of the activity had arrived at a given station within 3 to 6 hours after the detonation, with smll amounts continuing to arrive up to at least 12 hours after the detonation. Gamma dose rates at the shot atoll 1 hour after each shot were estimated to be as follows from data collected by this project and Rad Safe: Shot 18 1600 to 2900 r/hr along the northern islands, 160 to 630 r/nr on the eastern islands, and 15 to 43 rfnr along the southwest side of the atoll. Shot 2: 1100 to 4700 r/hr on the northwest islands close to ground zero and 2.4 to 14 r/hr on the remaining islands. Shot 3: 410 rfhr at Uncle, just west of ground zero, 10 to 125 rfhr on the north and northwest islands, and 0.8 to4.5 r where « Shot 4: else~ 160 to 440 r/hr on the north and northeast islands, and 0.1 to 23 r/hr elsewhere.