ABSTRACT A world-wide network of gummed film stations was established to monitor fallout following Operation Castle. Although meteorologi- cal data were poor, 3 general commection of tropospheric flow patterns with observed fallout.was evident, There was a tendency for debris to remain in trovical latitudes, with incursions into the temperate regions associated with meteorological disturbances of the predominantly zonal flow. As the season advanced, such incursions became more evident. Outside of the tropics, the southwestern United States received the greatest total fallout, about five times that received in Japan. The total world-wide fallout up to July 1, 195k, from thea Castle series estimated to be about qutside of the immediate test area, is of the total fission activity produced. The maximum fallout on any day at an individual station _in the United States, corrected to sampling day, was 200,000 d/m/ft?, It is concluded that the probability of early fallout in inhabited regions would be reduced by holding Pacific test series in the winter months. s