Using a collection system consisting of 3 graded filters placed in series, a study was conducted during the past year on the particle size characteristics of airborne fallout. Initial collections made during March and April 1964, consisted of fission products with an effective age of 440-550 days. Specific nuclides measured in these samples included Cl44_ Pr144 Sb125, Cs137, Zr95-Nb95, Mn54, Ru106-Rhl06 and Sr90. Either gamma spectrometry or radiochemistry was used for quantitative estimations. During the latter part of October and throughout November, 1964, a second series of collections were made which included fresh fission product material presumbably resulting from the atmospheric nuclear detonation in mainland China. In addition to the above nuclides, these samples were analyzedfor Ce141, Tel32-I132,I131, Ru103-Rh103, Ba104-Lal40 and Sr89. Differences in the particle size distribution of material on each filter were determined by optical microscopy. For older debris, at least 88% of the total activity was associated with particles having diameters of less than approximately 1.75 [mu]. In fresh fallout, certain radionuclides were initially associated with larger particles. With time, this distribution gradually decreased due to settling out of larger active particles. Data are presented on the degree of radionuclide fractionation which occurred in each size group.