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CO2 samples were collected on board container ships between Japan and Australia and between Japan and the United States from April 1984 to December 1991. They were then analyzed for their δ13C values with a precision of 0.02‰. The seasonal cycle of δ13C was clearly observable at latitudes from 55°N to the equator. Peak-to-peak amplitude of the average seasonal cycle of δ13C was about 0.8‰ at latitudes north of 40°N and decreased going southward to 0.2‰ at the equator. The comparison of the observed seasonal cycles of δ13C with those of the CO2 concentration suggests that the seasonal CO2 cycles observed at latitudes north of the equator are due to the seasonal change in terrestrial biospheric activities. However, air transport from the southern hemisphere is also thought to be partly responsible for the seasonal CO2 cycles at northern low latitudes. Values of δ13C decreased secularly at a rate of about −0.03‰/year, owing mainly to increased amounts of isotopically light CO2 produced in fossil fuel combustion and deforestation. Interannual variations of the long-term trend of δ13C in association with the El Niño–Southern Oscillation (ENSO) event and other factors were clearly observed, which were almost opposite in phase with those of the CO2 concentration. From the comparison of the observed long-term trend of δ13C with those calculated using a box diffusion model, it is suggested that an imbalance of the CO2 exchange between the atmosphere and the terrestrial biosphere could be responsible for such interannual variations.