sand like granules were not soluble in water on the microscope slide (Sh57). Most BRAVO activity probably remained with the fallout particles at the bottom of the cistern. The radioactivity in solution was probably in large part due to radioiodines because of their solubility. Extrapolating the 50 hour post detona- tion beta activity concentration back to 0.5 days post detonation, based on the decay characteristics of Bikini Ash components, results in a range of water gross beta activity concentrations of 0.003 to 0.05 Ci ml with a mean of 0.03 uci ml). At 0.5 days, a total of 1 Ci mn? of Bikini Ash gross beta activity was estimated from Table 5. Given the area of a cistern opening, this implied the average cistern contained 37 mn? of water if all the Bikini Ash activity was in the liquid phase. This was about 250 times greater than the observed water volume of the cisterns as previously indicated by Sharp (Sh57). If one assumes only 35% of the radioiodine activity in the liquid phase of cistern water, then an average cistern water content estimate of 0.3 m> of water results, about twice the water volume of cisterns as observed in late March, 1954, Assuming only 20% of the iodine activity in the liquid phase, the lowest estimate given by Kawahara (Ka66), and the appropriate activity of each iodine isotope in Bikini Ash at 50 hours post detonation leads to about the same estimate of cistern water content as that which was reported by Sharp and others (Sh57). Based on 1) Bikini Ash activity per unit area estimates given in Table 5, 2) a 20% release of iodine activity from fallout granules to cistern water and, 3) an average cistern water volume of 0.15 n°, the radtoiodine activity concentrations were estimated for cisterns located at Rongelap Island. A range of cistern water activity at Rongelap Island was estimated to be between 0.2 and 2 times the average values given in Table 6. The range was estimated on the basis of the range reported for the gross beta activity measured in Rongelap ° 24