An analysis of ozonesonde data for stations whose records go back at least a quarter of a century suggests that the number of intrusions of subtropical air into the midlatitude lower stratosphere has recently increased. These air masses, referred to here as ozone minima, contain low ozone concentrations over vertical depths of ~4 km, sufficient to reduce the total ozone column by 10--15%. The mixing ratios of other trace species, such as nitrous oxide (N2O), carbon monoxide (CO), and water vapor (H2O) recorded in midlatitudes by the NASA ER-2 aircraft, suggest that these intrusions originate either in the lower tropical stratosphere or in the upper tropical troposphere. Potential vorticity calculated from the European Center for Medium-Range Weather Forecasts (ECMWF) and National Center for Environmental Prediction (NCEP) date, have been used in conjunction with TOMS total ozone data to estimate the typical area covered by minima. Their contribution to the midlatitude ozone trend is discussed. TOVS data have also facilitated the identification of a prominent ozone minima source region in the subtropical western Atlantic, where the Atlantic storm track is particularly intense. Minima frequently advect over Europe, explaining the high abundance of these events observed by European ozonesonde stations, relative to ozonesonde data from Canada. ¿ 2000 American Geophysical Union