Abstract: Few published data are available to describe the size distribution of aerosols within the uranium mining environment. Activity size distribution of the radon progeny attached aerosols was measured at the Schwartzwalder uranium mine (Jefferson County, Colorado, USA) using the diffusion battery method in the summer of 1970. The measurements from five of eight sites in the mine were analyzed. The average activity median diameter was approximately 55.3 nm and ranged from 34.2 to 72.7 nm. The average geometric standard deviation was approximately 2.90. The geometric standard deviation computed from the samples collected during the evening (Sites 3 and 6) was significantly smaller than from samples collected in the daytime (Sites 5, 7, and 8). The results of Site 7 showed the greatest the geometric standard deviation, a significantly higher radon concentration and a low equilibrium condition.