This paper reports recent measurements of the shock piezoresistance coefficient of manganin (in wire and thin‐foil geometries) in high‐density polycrystalline alumina (Lucalox). For manganin in a wire geometry and for shock stresses below the Hugoniot elastic limit (HEL) of Lucalox, this coefficient was found to be much higher in Lucalox than in C‐7 epoxy, Hi‐D glass, and soda‐lime glass. The cause of the higher coefficient is postulated as the production of lattice defects in the manganin wire arising from large plastic deformation of the wire at compressive stresses below the HEL of Lucalox. For stresses above the HEL of Lucalox, the shock piezoresistance coefficient of manganin wire in Lucalox agrees quite well with that of manganin wire in other insulators. In a thin‐foil geometry in Lucalox, the coefficient was found to remain constant above and below the HEL of Lucalox.