From data collected at cruise levels by the five aircraft of the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) program during a 2-year period (September 1994 to August 1996), an accurate ozone climatology at 9--12 km altitude has been generated over the northern hemisphere (130 ¿W--140 ¿E; 0¿--80 ¿N), and down to 30 ¿S over South America and Africa. North of 35 ¿N, the distribution is dominated by the influence of ozone-rich air of stratospheric origin; farther south, ozone-poor air from the troposphere prevails. A classification based on a threshold of 100 ppbv of ozone is used to distinguish between the stratospheric and tropospheric components and to help in interpreting the data. Seasonal variations of ozone are observed in stratospheric air (maximum in spring, minimum in winter) and in tropospheric air (maximum in summer, minimum in autumn), related to the intensities of dynamic (tropopause variations; stratosphere/troposphere exchanges) and chemical (photochemistry) processes. The tropics exhibit variations that are heterogeneous in time and space, and reflect the influences of active photochemical processes, deep convection and biomass burning emissions. Ozone concentrations decrease with latitude in both the stratosphere and troposphere. A strong vertical gradient is found in stratospheric air but not in tropospheric air, which is much more homogeneous. The integration of data over characteristic geographical areas shows smaller concentrations over the Atlantic Ocean compared to the continents; this is related to the zonal variation of the polar front and the position of ridge/trough pressure systems. ¿ 1998 American Geophysical Union