Background mean ozone concentration estimates about 50% higher than were previously accepted come from an analysis of hitherto unreported balloon observations. Comparison of electrochemical and chemiluminescent ozonesondes, launched over the North American continent between 1963 and 1969, reveals a discrepancy between the two perceptions of mean ozone levels in the free troposphere, the electrochemical sondes giving higher values. Both sondes were normalized to the total ozone column measured by Dobson photometers, so that certain stratospheric corrections to the raw chemiluminescent soundings may be important to the tropospheric estimates. Internal evidence of the statistics of the two data sets suggests that the electrochemical sondes gave more reproducible results from flight to flight. We conclude that the Hering and Borden chemiluminescent ozonesonde statistics for the lower atmosphere are in doubt and prefer higher estimates of 0.04–0.10 μg/g for mean ozone mixing ratios of the middle troposphere above eastern North America and the Caribbean.