The O3 distribution calculated by a global three-dimensional chemistry transport model, TOMCAT, is compared to O3 data collected on Airbus A340 passenger aircraft as part of the Measurement of Ozone by Airbus In-Service Aircraft (MOZAIC) project. The model results are compared to seasonally averaged data at cruise altitudes in the upper troposphere and lower stratosphere and to individual vertical profiles collected over particular cities during takeoff and landing. In many regions, the model shows good agreement with these data, given the large variability encountered, particularly around the tropopause. There are also significant day to day variations in ozone, which, in the vertical profile data, TOMCAT captures well over northern midlatitude cities, especially in the upper troposphere and also in the winter lower troposphere. In the tropics, the MOZAIC data often shows a lot of structure, with thin layers of high O3 which are sometimes captured in the model. Possible reasons for this and other discrepancies are discussed. ¿ 1998 American Geophysical Union