The East African rift marks the northern boundary of the Nubian (West African) and Somalian (East African) plates, and has formed by horizontal stretching due to the separation of these plates(1). South of similar to 20 degrees S, any expression of deformation or seismicity due to the relative motion of these two distinct plates vanishes, although the boundary must continue until it intersects another plate boundary. The nearest such boundary is that of the Antarctic plate, marked by the Southwest Indian ridge. But previous analyses of plate-motion data have indicated no significant difference between Nubia-Antarctica and Somalia-Antarctica motion(2,3). Here we show, using: a large compilation of plate-motion data, that Nubia-Antarctica motion does differ from Somalia-Antarctica motion, and we determine a relative angular velocity of the two plates that has compact confidence limits. Our analysis places the pole of rotation near to the southern limit of African seismicity, implying that the southern part of the Nubian-Somalian plate boundary is a diffuse zone of convergence (up to similar to 2 mm yr(-1)), whereas up to similar to 6 mm yr(-1) of separation is accommodated across the East African rift-about half the separation rate of the slowest mid-ocean ridge.