ZIRCONIUM DIOXIDE (zirconia) has three allotropes: monoclinic, tetragonal and cubic. The transition between the first two involves a large volume expansion which prevents the refractory properties of pure zirconia being used in structural ceramics. The disruptive phase transformation can be suppressed by total stabilisation in the cubic form, but it is generally recognised that the most useful mechanical properties are obtained in multiphase material known as partially-stabilised zirconia (PSZ). Garvie and Nicholson1 have demonstrated that a fine-scale precipitate of monoclinic zirconia in a cubic stabilised matrix enhances the strength of PSZ. Here we report that a dispersion of metastable tetragonal zirconia in cubic zirconia can also be achieved, and that this gives rise to another, more powerful, strengthening mechanism.