This study investigates the distribution and the sources of ozone (O3) observed in the uppermost troposphere and lowermost stratosphere within the framework of the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) program. The O3 measurement data taken aboard five commercial aircraft during a 1-year period are combined with 8-day three-dimensional back trajectories ending at the aircraft positions. Potential vorticity along the trajectories is used to form subsets of the data taken in the troposphere, in the stratosphere, and in stratosphere-troposphere exchange flows. A measured O3 value is then attributed to the entire path of the corresponding trajectory. A statistical analysis of this data set extends previous global MOZAIC O3 climatologies which are restricted to the major flight corridors. Furthermore, it allows source region identification of the measured O3. It is found that the North American and Eurasian continental boundary layers are the major sources of upper tropospheric O3 in spring and summer, while the African boundary layer is the greatest O3 source in fall and winter. The seasonal O3 variation is strongest in air masses exported from central Asia, followed by North America. The central Asian O3 maximum is higher and occurs earlier in the year than the North American maximum, which possibly indicates a great influence from biomass burning emissions in Asia. In the lowermost stratosphere the dominant structures in the O3 distribution are a strong north-south gradient and a wave-like pattern with O3 maxima over North America and eastern Asia. O3 concentrations in stratosphere-troposphere exchange flows are higher than in tropospheric air throughout the year and thus provide an important O3 source for the troposphere. However, our analysis suggests that photochemical O3 formation is more important than stratosphere-troposphere exchange during most of the year. ¿ 2001 American Geophysical Union