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The purpose of this research was to examine iso-dose rate contours and cloud arrival time data from atmospheric nuclear tests conducted at the Nevada Test Site to validate size distributions currently used in fallout prediction models or to propose new size distributions. Results were obtained from studies of eight air, nine tower, two surface and two shallow subsurface bursts. Yields ranged from 0.3 to 74 kilotons and heights of burst ranged from - 67 to 1500 feet. The tests examined are tabulated in Appendix A. Iso-dose rate contours from U.S. atmospheric nuclear tests (9) were digitized and then integrated numerically to obtain information on the fraction of activity grounded with time. The parameters of the best-fit lognormal and best-fit power- law particle-size distributions were then found for each burst studied. There was insufficient fallout from tower and air bursts to make conclusive determinations about the size distributions produced by these classes of bursts. For surface and shallow subsurface bursts the debris may be characterized by a lognormal distribution, by a power-law r to the -n power distribution, or by a linear combination of these or possibly other distribution functions. While no definitive distribution function was established, a linear combination of lognormal distributions seems to hold the most promise. In addition, a lower bound of 2210 r-sq. mi/hr-kt was established for the source normalization constant.