Shudofsky [1985> has established that earthquakes associated with the East African rift system have well-constrained focal depths as great as 25--30 km. Using published heat flow measurements as a guide to the local geotherm, we find through simple stress envelope calculations that the deepest earthquakes probably occur in the lower crust in a region where the lithosphere is strong. These results are at odds with the commonly held idea that seismicity in zones of continental extension is limited to the shallow upper crust because of elevated temperatures accompanying lithospheric thinning. Any model of the rifting process in East Africa must account for the fact that these regions exhibit considerable strength down to lower crustal levels. ¿American Geophysical Union 1987