Rutile-structured SnO2, PbO2, and RuO2 have long been known to transform to cubic high-pressure phases, for which a fluorite structure has been assumed. Rietveld refinement results from x-ray diffraction studies indicated that these phases have a modified fluorite structure (space group Pa(3) over bar). Thus, for metal dioxides, all known cubic, postrutile phases have the Pa(3) over bar structure, thereby providing experimental examples of the high-pressure structure predicted from theoretical calculations for stishovite (rutile-structured silica). High-pressure transitions in stishovite may have profound implications for the geochemistry of the core-mantle boundary.