Using balloon-borne condensation nuclei counters at Laramie, Wyoming (41¿N), a large increase in what appear to be small sulfuric acid droplets above 29-km altitude was detected on January 28, 1983. Simultaneous reductions in electrical conductivity in the layer supported the aerosol observations. The air sample associated with this event appears to have originated in the north polar region where a stratospheric warming episode was taking place. It is believed that the substantial warming in the region may have been sufficient to vaporize resident sulfuric acid aerosol which were mainly derived from the El Chichon volcanic eruption 10 months earlier and transported to the polar region during winter. It is further believed that subsequent rapid cooling of the vapor during transport to the latitude of Laramie resulted in the nucleation of the small droplets, forming an extensive sulfuric acid fog above 29 km. The upper boundary of the cloud was not determined but extended to at least the maximum altitude attained (32.5 km). Longitudinal spreading appears to have been rapid, and remnants of the fog were detected as far south as 32¿N in May and persisted throughout the year at Laramie. A similar phenomenon is believed to occur in the Antarctic stratosphere as revealed in a measurement there in the southern hemisphere spring (October 1983).