The stable isotopic concentration (18O and 2H) of water in phreatic aquifers in the Sahelo-Sudanese zone of Africa is representative of the original mean rainfall contributing to groundwater recharge. The isotopic decrease of 18O in the rainwater and groundwater (-0.084% 100 km-1) showed an east-west gradient. This suggests a supply of water vapor transported by the zonal flows East African Jet, Tropical Easterly Jet, and Easterly Waves, thus increasing the contribution from the evaporating surfaces of the Indian Ocean in relation to that of the Guinean monsoon. The gradient also indicates the importance of atmospheric recycling of continental water through evapotranspiration. A model of conservation of the isotopic masses is presented which takes into account the return of continental vapor toward the rain-giving clouds. The remaining fraction of water vapor during the east-west transit between Djibouti and Dakar is thus determined by evapotranspiration. ¿ American Geophysical Union 1992