Concentrations of particles having radii <0.1 &mgr;m (Aitken particles) in the stratosphere immediately above the tropopause were measured with a condensation nucleus counter during horizontal flights of a jet aircraft. Order-of-magnitude variations in the concentrations were observed which may have resulted from the interaction of gravity waves with strata of the particles. Since the concentrations of Aitken particles are ususally an order of magnitude or more smaller than those of ions in the stratosphere, it is likely that at most only a small fraction of such ions play an important role in the production of stratospheric particles.