ABSTRACT Twenty-seven penetrations o3 from multimegatonrange detonations were rade at tines r m 20 to 78 minutes after detonation and at altitudes ranging from 20,000 to 50,000 feet. Sixtesn of these penetrations were earlier than 45 mimites and seven were earlier than 30 mimutes. Maximm radiation dose rates as high as G00 r/br were encountered, © and several flights yielded total radiation doses to ths crew of l5 r, as measured by film badges, and 35 to 40 r, as maasured by instrucenta~ tion more sensitive to lowerergy radiation, It was found that the average radiation dose rate in the mushroom of the cloud from a l00-percent-fission-yield detonation would be: BD: 1.0 x 10°5¢2+? Where: D= Average dose rate, r/hbr. t = Time after detonation, mimites. This relationship holds for times from 3 to & minutes after detonation. The average dose rate in the stem of the cloud from water-surface bursts was found to be less than the dose rate in the mushroom by a factor of fron five to ten. The radiation dose rate in the cloud is independent of yield, but is proportional to the ratio of fission yield to total yield, In a high tropopause area, a flight through a cloud fren a 100percent-fission~yield miltimegaton-range weapon in a high-performance aircraft may be made at 45,000 feet at a time of 20 minutes after dato-~ nation, The average mission dose of this flight would be 25 r, At 30,000 feet, a penstration of the stem of the cloud may be made as early as 10 minutes after detonation with a radiation dose of the sams magnitudes. The dosage received on the return to base flight dus to contamina~ tion on the aircraft (B-57B) was found to be about 15 percent of the total mission dose for flights lasting about 50 minutes after the cloud penetration,