Two gravity and magnetic intensity profiles across the Vema fracture zone in the crestal region of the mid-Atlantic ridge indicate that the southern flank of the fracture valley is underlain by a zone of excess mass. Also, the 'south wall' of the fracture zone is partially 'nonmagnetic' and may be composed of rocks of induced magnetization, low in remanence compared to adjacent basaltic crust. The regions of excess mass and low magnetization probably reflect upper-crustal ultramafic and serpentinized ultramafic rocks located along the fracture zone. Similar rocks may underlie the north flank of the fracture valley as well.