The surface of Venus is so hot that the lower crust may be weak enough to allow decoupling of mantle and crust. An analytic model of such decoupling assumes that the shallow mantle forms the top boundary layers of large scale mantle convection cells. Crustal flow is driven by the motion of the mantle and by topographically induced pressure gradients. The model predicts that the lowest lowlands are sites of mantle upwelling and thinner than average crust. Highlands are places where mantle downwells and the crust is thick. Surface heat flow is inversely correlated with elevation, consistent with recent estimates of brittle layer thickness variations on Venus. If the average crustal thickness is about 20 km then the average lower crustal viscosity must be close to 1018 Pa s to allow decoupling. The observed amplitude of geoid highs over highlands requires an Earth-like increase in mantle viscosity with depth. ¿ American Geophysical Union 1992