A global climatology of total column ozone has been computed from 4 years of daily observations by the total ozone mapping spectrometer aboard the Nimbus 7 polar-orbiting satellite. Observations were made at local noon; no observations are available in polar regions during the polar night. The orbital data have been area averaged onto a regular 5¿ by 5¿ latitude-longitude grid. The global coverage is more than 90% complete during the last 3 observing years (October 1979 to September 1982) and approximately 70% complete during the first year (October 1978 to September 1979). Global maps of the temporal mean and rms variance and the amplitude and phase of the annual and semiannual harmonics are presented. Similar analysis of the zonal mean values are given along with a time-latitude section of the zonal-mean total ozone. This climatology should be useful both for validating numerical models and for inspiring theoretical analyses.