It is proposed that western Ishtar Terra formed due to compression and crustal thickening above a cylindrical mantle downwelling. A model for crustal deformation due to downwelling sucessfully reproduces many observed characteristics of western Ishtar. Although axisymmetric downwellings occur in numerical models of constant-viscosity mantle convection, there is no evidence for their existence in Earth's mantle, where downwellings are sheet-like. Either modes of downflow in Venus and Earth are fundamentally different, or differences in near-surface conditions and material behavior selectively emphasize surface expressions of the different downwelling modes. ¿ American Geophysical Union 1990