Balloonborne measurements of stratospheric clouds using optical particle counters in the Arctic and the Antarctic are compared in terms of particle size distribution. There appear to be two distinct classes of particles, a small mode (r≥0.2 &mgr;m) in which at least half of the available condensation nuclei have grown and a large mode (r≈2--3 &mgr;m) in which fewer than 1% of the available condensation nuclei have grown. The latter particles appear in thin layers (0.1 to 0.3 km) while the former appear in relatively thicker layers (2 to 5 km). Temperatures dictate a nitric acid trihydrate composition for most of the layers and the inferred HNO3 vapor in the large particles is comparable to that available. While the small particle mode is prabably related to fast cooling events such as those associated with mountain lee waves, the origin of the thin layers of large particles in not apparent. ¿ American Geophysical Union 1989