ABSTRACT A world-wide network of gummed film stations was established to monitor fallout following Operation Castle. Although meteorologi- cal data were poor, a general connection of tropospheric flow patterns with observed fallout.wes evident. There was a tendency for debris to remain in tropical 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 Uni ted States received the greatest total fallout, about five times that received in Japan. The total world-wide fallout up to July 1, 195k, from the Castle series qutside of the immediate test area, is estimated to be about of the total fission activity produced. The maximum fallout on any day at an individual station,in the United States, corrected to samoling day, was 200,000 d/m/ft2, a ret It is concludedthat the probability of early fallout in inhabited regions would be reduced by holding Pacific test series in the winter months. :