The onset of the monsoon system over West Africa is linked to the northward migration of the Inter-Tropical Convergence Zone (ITCZ) during the northern spring and summer. By using daily gridded rainfall data and NCEP/NCAR wind reanalyses over the period 1968--1990, we show that this migration is characterized by an abrupt latitudinal shift of the ITCZ in late June from a quasi-stationary location at 5N in May-June to another quasi-stationary location at 10N in July-August. A composite analysis based on the shift dates shows that this northward shift is associated with the occurrence of a westward-travelling monsoon depression pattern over the Sahel with characteristic periodicities of 20--40 days. ¿ 2000 American Geophysical Union