A persistent minumum was observed in the ozone partial pressure profile at 39 mb over Yorkton, Saskatchewan (51¿ N, 102¿ W), on each of 27 ozonesonde flights between August 7 and 31, 1976. This notch in the profile has an average vertical extent of 3 km and an average decrease in the partial pressure or mixing ratio of 35% below the adjacent maxima. The persistence and spatial extent of this phenomenon were investigated. An ozone-aerosol flight at Laramie. Wyoming (41¿ N, 106¿ W), on August 6 also showed a minimum in both the ozone and aerosol profiles. It is demonstrated that this is a frequent phenomenon of the stratosphere which occurs at mid-latitudes in late summer. The phenomenon occurs within a broader layer of very light winds situated between the westerlies below and easterlies above. From the evidence available it is not possible to choose between differential quasi-horizontal advection and a photochemical sink as the mechanism responsible for producing the layer.