Concentrations and the Δ13C of CO2 were measured on air in the lower portion of the atmospheric boundary layer in the Amazon basin. Three major trends were observed: (1) a diurnal cycle with an amplitude of 70 to 150 ppm for concentration and 3 to 6% for Δ13C, (2) a vertical gradient of CO2 and Δ13C in forest air, and (3) different 13C composition for CO2 in air over the river and in the forest. The diurnal changes in CO2 concentration and Δ13C of the CO2 resulted from diurnal cycles in boundary layer mixing and photosynthetic uptake of CO2. Photosynthetic uptake of CO2 increased the Δ13C to levels up to ~0.6% higher than tropospheric values in forest air samples collected during the afternoon at heights >7 m. An observed 5% decrease in Δi13C of CO2 in forest air between canopy height and ground level indicates that photosynthetic uptake of biogenic CO2 is an important factor in determining the Δ13C of plants growing in the Amazonian rain forest. The observed nighttime increase in CO2 concentration and Δ13C decrease differed for air within the forest canopy versus air over the river. The Δ13C values of the CO2 sources for air in the forest and over the river were -28% and -22%, respectively. The results indicate that soil and plant respiration were the primary sources of CO2 input to air in the forest, whereas degassing of CO2 from the Amazon River is the likely source of CO2 input to air over the river. ¿ American Geophysical Union 1989