The assertion of cratonic stability put forward in the model for deep continental structure can be tested by examining upper mantle structure beneath the Tanzania Craton, which lies within a tectonically active region in east Africa. Tomographic inversions of about 1200 teleseismic P and S travel times indicate that high-velocity lithosphere beneath the Tanzania Craton extends to a depth of at least 200 km and possibly to 300 or 350 km. Based on the thickness of mantle lithosphere beneath Archean cratons elsewhere, it appears that the mantle lithosphere of the Tanzania Craton has not been extensively disrupted by the Cenozoic tectonism in east Africa, thus corroborating the assertion of cratonic stability in the model for deep continental structure. The presence of thick, high-velocity structure beneath the Tanzania Craton implies relatively low temperatures within the cratonic mantle lithosphere, consistent with relatively low surface heat flow. The thick cratonic keel is surrounded by low seismic velocity regions beneath the east African rifts that extend to depths below 400 km. Our models show a shear velocity contrast between the cratonic lithosphere and the uppermost mantle beneath the eastern branch of the rift system of about 5% to 6%, but from resolution experiments we infer that this contrast could be underestimated by as much as a factor of 1.5. We attribute about half of this velocity contrast to the depleted composition of the cratonic keel and the other half to thermal alteration of upper mantle beneath the rifts. Low-density structures that may be required to provide buoyant support for the elevation of the Tanzania Craton must reside at depths greater than about 300--350 km. ¿ 1998 American Geophysical Union