Shear waves have been measured in shock-induced high-pressure phases of Carrara marble (polycrystalline calcite). The shear modulus decrease associated with the calcite-CaCO3(II) transition is comparable to that observed under hydrostatic loading. Beyond this transition, the shear modulus in the shocked marble increases with compression, even with the occurrence of the CaCO3(II)-CaCO3(III) transition. Longitudinal wave speeds over the entire compression range examined are in reasonable agreement with previous shock wave and hydrostatic measurements in polycrystalline CaCO3. A preliminary determination of the bulk modulus from the shear and longitudinal wave speeds in the shocked marble is in reasonable agreement with hydrostatic measurements on polycrystalline and single crystal calcite, suggesting that the transformation to CaCO3(III) may be complete during shock compression. The usefulness of shear wave measurements in studying shock-induced phase transformation in solids is indicated. ¿ American Geophysical Union 1989