A climatology of the vertical ozone distribution in the troposphere is obtained by balloon-borne Brewer-Mast sondes at the Observatoire de Haute Provence (OHP), southern France (44¿N, 6¿E, 700 m above sea level), during the period 1984 to 1990. The tropospheric ozone seasonal variation is characterized by a large maximum in spring and summer. A comparison to other ozone sounding stations gives evidence for a meridional ozone gradient in the middle and upper troposphere in Western Europe, with larger ozone values at Uccle (51¿N, 4¿E) and J¿lich (50¿N, 6¿E) than at the OHP (44¿N, 6¿E). A statistical analysis of ozone concentrations together with potential vorticity, a tracer of stratospheric air masses, shows a partial but significant correlation between both variables (r=0.40, significance >99.9%), implying a noticeable impact of stratosphere-troposphere exchange on the ozone variability. Concerning the spring/summer maximum of tropospheric ozone, it is shown to be caused both by ozone transfer from the stratosphere and by in situ photochemical ozone production. A climatology of the ozone/potential vorticity ratio is established for the OHP and its altitude and seasonal dependence is given. Finally, it is shown that the interannual variability of potential vorticity can cause dynamically induced trends of the ozone concentrations, which have to be taken into account to determine accurately the relative part due to human activity. ¿ American Geophysical Union 1994