An airborne monitoring system has been used to observe fine-scale filamentation in the ozone field between 370 and 390 K isentropic levels. The Australian long-range, high-altitude research aircraft, the Egrett, was used to record multiple atmospheric properties with a high spatial resolution. Toward the end of 5 June 2000, an intrusion of low-ozone air was recorded over Aberystwyth, between 360 and 390 K. On the same day, a Rossby wave was observed to be breaking over western Europe with both poleward and equatorward components. We suggest that the low ozone at ~380 K was caused by the poleward component of the breaking Rossby wave, stretching from southwestern Ireland to Iceland. On the eastern flank of the subtropical intrusion the in situ ozone measurements observed by the Egrett showed distinctive step and trough structures that were not expected from conventional model analyses. Using Reverse Domain-Filled (RDF) trajectory analyses to produce a fine-scale view of potential vorticity, we show how breaking of small-scale Rossby waves along the flank of the main wave over the northeastern United States, followed by a strong zonal compression, produced fine-scale filaments responsible for the structures observed in the Egrett ozone measurements.