Examination of 256 particles in the 2--50 μm size range obtained from stratospheric impaction collectors has shown that most of them fit into one of three groups on a simple plot of Mg/Si versus Al/Si energy dispersive X ray peak ratios. The particles within a given group exhibit strong correlations in composition and mineralogy to one another, and the groups appear to largely consist either of earth crustal, manmade, or extraterrestrial materials. The temporal variability and relatively flat size distribution of material in the earth crustal group, in particular, suggests an aircraft-related, as opposed to stratospheric, source for these particles.