An extensive layer of small particles (r>0.01 &mgr;m) was observed in association with the passage of a jet aircraft at 23 km altitude. The data are used to estimate the small particle emission index and the source strength of such aircraft at stratospheric altitudes. The impact of such flights on the stratospheic particle population is probably negligible; however, when the results are applied to the currently projected Concorde SST fleet in the stratosphere and the commercial jet fleet in the upper troposphere both are estimated to contribute nonnegligible equilibrium concentrations of such particles to their respective altitude regimes.