Data obtained from the Global Atmospheric Sampling Program (GASP) during the late 1970s are used to evaluate the hypothesis that particulate emissions by aircraft are a significant source for the upper tropospheric/lower stratospheric fine and ultrafine aerosol. Measurements of condensation nuclei (CN) were acquired above 6 km mean sea level as part of GASP from October 1977 through June 1979, using automated packages on up to four commercial airliners simultaneously flying their normal daily routes. These routes spanned much of the midlatitude Northern Hemisphere, with the notable exceptions of the former Soviet Union and China. The GASP measurements are analyzed to determine the vertical, geographical, and seasonal distribution of upper tropospheric/lower stratospheric CN in clear air, spanning an altitude range from several kilometers above to several kilometers below the tropopause. The general lack of vertical gradients in median CN mixing ratio at altitudes in the troposphere suggests that CN in the clear troposphere are generally well mixed. CN mixing ratios decrease with distance above the tropopause, consistent with a tropospheric source for stratospheric CN. Median CN mixing ratios for selected regions were compared to the Northern Hemisphere median, by altitude and season. Regional median CN mixing ratios can differ from the hemispheric median by more than a factor of 2 but never an order of magnitude. The lack of vertical variation of CN concentrations in the troposphere is not at all consistent with the vertical variation of fuel consumed by aircraft, which peaks at altitudes near the tropopause. The observations of CN show some regional medians exceeding Northern Hemisphere medians in some seasons and not others, but the regions with the highest medians are not always those with the highest aircraft fuel consumption. We conclude that aircraft emissions are not a major component of the global clear air background CN measured during GASP.