An increase of 3% in the ''d'' value of the isotopic composition of precipitation over the Amazon Basin suggests that an isotopically fractionated evapotranspiration flux contributes to the atmospheric water balance over the region. A steady state evapotranspiration model for the Amazon Basin was developed, and it is shown that the observed increase in the ''d'' value corresponds to a situation where 20--40% of the total evapotranspiration flux is accompanied by an isotopic fractionation, such as by evaporation from an open water surface. The site of this fractionation, whether by evaporation from the flood plains, canopy interception or lakes, cannot be resolved with the presently available data set. ¿American Geophysical Union 1991