ABSTRACT A world-wide network of gummed film stations was established to monitor fallout following Operation Castle, Although meteorological data were poor, a general connection of troroscheric flow patterns with observed fallout was evident. There was a tendency for debris to remain in tronical latitudes, with incursions into the temperate regions associated with metecrological disturbances of the predominantly zonal flow. As the season advanced, such incursions became more evident. Outside of the tropics, the southwestern Uni ted States received the greatesttotal fallout,about five times that received in Japan. ... . The maximum fallout. on any day at an individual station in the | United States, corrected to sampling day, was 200,000 a/m/tt2, It is concluded that the probability of early fallout in inhabited regions would be reduced by holding Pacific test series in the winter months.