Unusual aerosol layers in the upper troposphere were observed on seven consecutive balloon soundings between 7 March and 17 June 1991, at Laramie, Wyoming (41¿N, 106¿W). The layers varied in thickness from 1 to 4 km and the top of the layer was always at or near the tropopause. Aerosol concentrations for the size range 0.15 to 1.0 &mgr;m radius were increased by factors of 5 to 10 over what is usual at those altitudes. Compared with the previous four year record, these seven soundings represent the highest sustained concentrations observed between 8 and 11 km, with the sounding on March 25 representing the highest concentrations observed. Although the origin of these unusual aerosol layers is not definite, trajectory calculations and impactor measurements indicate that, in the absence of an alternate explanation the oil fires in Kuwait are the likely source.