Four long seismic refraction profiles were shot in the Jordon--Dead Sea--Gulf of Flat graben and adjacent areas. From the arrival times of the Pg, Pn, and PMP phases, models of the crust and upper mantle were computed. The models show a crustal thickness of 30 km along the garben with a considerable thinning to the south along the Gulf of Elat. Between the Dead Sea, Red Sea, and the Mediterranean, the crust thins rapidly to a thickness of 20 km while the overly ing sediments thicken considerably. The results indicate an upper mantle upwelling in the graben area, connected to the Red Sea spreading mechanism, and the presence of a thin, possibly oceanic crust covered by a thick wedge of Mesozoic and Tertiary sediments underneath the Northern Negev of Israel and probably extending offshore to the Eastern Mediterranean.