An analysis of 531 Umkehr observations (1964–1966) taken at Pretoria, Republic of South Africa, shows that ozone concentration increases rapidly above 16 km, the maximum concentration occurring at 25.3 km. Seasonal changes in ozone concentration observed in the lower stratosphere and in part in the middle stratosphere are in phase with the total ozone changes, with a maximum during the late spring. Above 33 km the concentration changes very little, increasing only slightly during the summer and decreasing slightly during the winter, thus indicating the dominance of photochemical processes. The ozone between about 24 and 33 km responds only slightly to the spring increase in total ozone, but it increases during the summer months. The amplitude of these changes is much smaller than it is in the lower stratosphere. The layer between 24 and 28 km (sometimes up to 33 km) seems to act as a transition zone between the upper stratospheric (photochemical) and lower stratospheric (circulational) regimes of changes in the ozone concentration. Comparison of the vertical profile at Pretoria with the average profile at 25°N shows that the northern hemisphere is only 2% richer in ozone below 10 km and slightly poorer in ozone between 15 and 33 km. Correlations of the ozone concentrations in nine atmospheric layers, as well as the correlations between the layer concentrations and the total ozone, support the view that changes in the vertical distribution can be described adequately by considering changes in four broad layers: (1) the troposphere, (2) the lower stratosphere and up to about 25 km, (3) the transition zone starting at 25 km to 35±2 km, and (4) the region usually above 33–37 km.