An aerosol layer has been identified by a stratospheric balloon and aircraft aerosol collection program. Measurements of horizontal extension and vertical distribution indicate that this layer is a world-wide phenomenon, displaying little variation with time and latitude. The particles in this layer range in size between 0.1 and 2 µ in radius, are water soluble, and contain sulfur as the predominant constituent. It is most likely that they are formed within the stratosphere.