Between March 1977 and February 1982, 517 samples of air were collected in 5 1 glass flasks at four stations in the northern hemisphere near the Pacific ocean and at the South Pole. First, the CO2 concentration of each sample was determined by nondispersive infrared gas analysis, and then the 13C/12C and 18O/16O ratios of the cryogenic extracted CO2 were determined with a triple collector mass spectrometer. For each station the secular trend and seasonal variation in 13C/12C ratio have been established as a function of CO2 concentration. The seasonally adjusted 13C/12C ratio is found to have decreased at a rate of about 0.02% per ppm increase in the seasonally adjusted CO2 concentration and to vary with latitude as expected if CO2 is being released by fossil fuel combustion in the northern hemisphere and from ocean water near the equator. At the three northernmost stations (La Jolla, California, at 33 N and two Hawaiian stations near 19¿N) the 13C/12C ratio of the CO2 added to and withdrawn from the atmosphere during one annual cycle is circa-29% with respect to standard PDB, as expected from the exchange of atmospheric CO2 with the carbon of terrestrial plants. Near the equator at Fanning Island(4¿N) the similarly computed 13C/12C ratio is -21% while at the South Pole it is about -13%. This suggests that the seasonal variation in the southern hemisphere and tropics is partially at result of oceanic CO2 exchange.