An airborne ozone lidar has been used to observe layering in the ozone field between 350 and 400 K northward of the subtropical jet stream. An ozone-poor layer between 370 and 400 K was observed over western Europe on 21 April 1999. On this day a Rossby wave was observed to be breaking over the eastern North Atlantic and western Europe, with both poleward and equatorward components. We propose that the low ozone at ~400 K was caused by a poleward breaking Rossby wave stretching from Spain to the south coast of Britain. Isentropic back-trajectory calculations suggest a tropical upper tropospheric origin for the air within this wave, over North America. We suggest that Rossby waves generate layering in the lower stratosphere because of vertical gradients in the background wind fields, which govern the growth in amplitude and breaking characteristics of the waves.