Uranium aerosols generated during normal yellowcake packaging operations were sampled at four uranium mills. Samplers located in the packaging area were operated before, during and after drums of yellowcake were filled and sealed. Median aerosol concentrations in the packaging areas ranged from 0.04 micrograms U/1 to 0.34 micrograms U/1. The aerosols were heterogeneous and included a broad range of particle sizes such that 14% to 76% (by weight) of the airborne uranium was in particles with aerodynamic diameters greater than 12 micron. Air concentrations and particle-size distributions varied with time as larger particles settled or more aerosols were suspended. Aerosol characteristics could often be related to individual packaging steps. The results show that appreciable amounts of airborne uranium would be expected to deposit in the nasopharyngeal compartment of the respiratory tract if inhaled by a worker not wearing respiratory protection.