We present evidence for stratospheric mass transport into, and remaining in, the troposphere in an intense midlatitude cyclone. Mesoscale forecast model analysis fields from the Mesoscale Analysis and Prediction System were compared with total ozone observations from the Total Ozone Measurement Spectrometer. Coupled with parcel back trajectory calculations, the analyses suggest that two mechanisms contributed to the mass exchange: (1) A region of dynamically induced exchange occurred on the cyclone's southern edge. Parcels originally in the stratosphere crossed the jet core and experienced dilution by turbulent mixing with tropospheric air. (2) Diabatic effects reduced parcel potential vorticity (PV) for trajectories traversing precipitation regions, resulting in a PV hole signature in the cyclone center. Air with values of ozone and water vapor typical of the lower stratosphere was left in the troposphere. The strength of the latter process may be atypical. These results, combined with other research, suggest that precipitation-induced diabatic effects can significantly modify (either decreasing or increasing) parcel potential vorticity, depending on parcel trajectory configuration with respect to jet core and maximum heating regions. The diabatic heating effect on stratosphere-troposphere exchange (STE) is more important to tropopause erosion than to altering parcel trajectories. In addition, these results underscore the importance of using not only PV but also chemical constituents for diagnoses of STE. ¿ 2001 American Geophysical Union