Between March 1979 and June 1980, air was collected on the Hawaii-Tahiti shuttle expedition of the Global Atmospheric Research Program. A north-south track was repeatedly sampled and the air analyzed to provide eight sets of data on the concentration, 13C/12C and 18O/16O ratios of atmospheric carbon dioxide between 20¿N and 17¿S latitude near 155¿W. In the northern hemisphere the 13C/12C ratio of the CO2 seasonally added to and withdrawn from the atmosphere was found to be nearly constant at circa 30% less than standard PDB. This ratio is consistent with photosynthesis and respiration of terrestrial vegetation as the cause of the seasonal cycle. The ratio was progressively less negative southward, reaching about -13% at 17¿S. This shift in ratio suggests that air-sea exchange is important in producing the seasonal variation in the southern hemisphere, since this exchange probably occurs with little kinetic isotope fractionation. The seasonally adjusted 13C/12C ratio of atmospheric CO2 increased steadily southward from -7.5% near 16¿N to -7.47% near 17¿S, as expected from a predominantly northern hemisphere source of a CO2 from fossil fuel combustion and a barrier to atmospheric mixing in the tropics. A peak in CO2 concentration near the equator was not found in the north-south trend in 3C/12C ratio. The lack of an isotopic peak suggests that the high concentrations of CO2 near the equator is produced by the release of CO2 from seawater and is not related to biological activity of land vegetation.