The two-dimensional structure of a tropopause fold that formed on the western flank of a cut-off cyclone over the western United States is examined through time series of vertical ozone profiles made using the NOAA Aeronomy Laboratory's Differential Absorption Lidar (DIAL) system at Fritz Peak Observatory (39.9 ¿N, 105.3 ¿W, 2.68 km above sea level). The observations are compared to calculated potential vorticity fields, and the net influx of ozone to the troposphere is estimated to be ~5.5¿1032 molecules assuming that the fold is irreversibly mixed into the troposphere below 8.5 km. ¿ American Geophysical Union 1996