During a 1976 rural ozone field study conducted in McKee, Kentucky, and Busick, North Carolina, simultaneous measurements of ozone and beryllium-7, a tracer of stratospheric air, were obtained. The purpose of this study was to assess the importance of naturally produced stratospheric ozone on ground-level ozone concentrations observed in rural areas in the Eastern United States. It is currently accepted that the stratospheric contribution to the surface ozone is about 30-50 ppb, and that the occurrence of this is confined to the front part of high-pressure systems immediately behind a cold front. Furthermore, it is thought that the stratospheric contribution is negligible on the back side of the high-pressure system where higher levels of ozone occur. This study seriously questions both of these hypotheses because the highest levels of beryllium-7 were observed on the back side and not on the front side of high-pressure systems.