Measurements by an ECC ozonesonde launched from Aberystwyth (52.4 ¿N, -4.1 ¿E) in July show ozone concentrations decreasing steadily from a warm frontal surface to the tropopause, cumulating in a layer ~0.5 km deep with near-zero ozone concentrations at 12 km. Such features have previously been detected by lidar but have not been reported in ozonesonde data at mid-latitudes; they have, however, been found in ozonesonde profiles above the equatorial Pacific. We examine three possible hypotheses for the origin of the ozone-free air: in situ destruction by cirrus clouds, rapid transport from the marine boundary layer in the extratropics and long-range transport from the tropics. We conclude that the ozone-poor air observed on this day could only have resulted from long-range transport from the tropics. ¿ 1998 American Geophysical Union