On the 1978 Global Atmospheric Measurements Experiment of Tropospheric Aerosols and Gases (Gametag) flights, 201 measurements of the tropospheric concentration of SO2 were made over a latitude range 57¿S to 70¿N. The area sampled included the central and the southern Pacific Ocean and the western section of the United States and Canada. Sulfur dioxide levels averaged 89¿69 pptv in the boundary layer and 122¿85 pptv in the free troposphere in the northern hemisphere. In the southern hemisphere, SO2 concentrations averaged 57¿18 pptv in the boundary layer and 90¿21 pptv in the free troposphere. The mean concentration of the continental data was 112¿79 pptv in the boundary layer and 160¿100 pptv in the free troposphere. The SO2 marine values were 54¿19 pptv in the boundary layer and 85¿28 pptv in the free troposphere. From a simple chemical model we conclude that a significant amount of background SO2 may originate from the oxidation of OCS.