the sampler aircraft, and to collect a sample of a primary sampling aircraft which aborted, The rate of decrease of radiation intensity within the cloud as a function of time after burst appeared to be typical of CASTLE bomb clouds. The levels at corresponding times, however, were substantially lower (about a factor of 50) at a below an indicated pressure altitude of 50,000 feet. From the control aircraft, numerous strokes of lightning were observed at approximately H f 15 to 20 minutes within the stem below the cloud base. feet. The latter was estimated to be about 25,000 This observation, as well as the pure white appearance of the stem, suggest: that no radioactive debris was present within it. On the other hand, as the control airplane performed a climbing appfoach on a westerly heading toward a persistent layer of bomb cloud with a base about 55,000 feet true altitude, it found patches of debris having approximately CASTLE~like radiation intensities. This finding, as well as the radiation field observed bela this layer and its physical appearance, suggests that a "normal" concentration of radioactive material 1sapproached only at altitudes above 50,000 - 55,000 feet ii It is concluded that sampling air~ craft with at least 10,000 feet higher altitude capability than the B-57B mst be used in order to collect satisfactory samples from high yield bursts fired at altitudes of 5,000 feet or higher. == \b