The bulk modulus of MgCO3 magnesite has been measured to 20 GPa at room temperature using X-ray powder diffraction. Since the response to pressure is primarily compression of the MO6 octahedron, magnesite is substantially less compressible than Ca-carbonates of similar structure type. Using the Birch equation of state, with KT' of 4.0, we obtain KT of 142 (¿9) GPa at room temperature. The full Birch-Murnaghan equation of state yields KT of 151 (¿7) GPA and KT' of 2.5. These results, together with other thermodynamic data, have been applied to calculate the stability of magnesite under lower mantle conditions. Assuming lower-mantle saturation in (Mg,Fe)O, magnesite should be stable to temperatures well in excess of 6000 K at the core-mantle boundary, i.e., at least up to the solidus of peridotitic or pyroxenitic compositions. This confirms that magnesite is a stable host for carbon in the mantle. ¿ American Geophysical Union 1993