Creep tests have been performed on 21 mole% yttria fully stabilized zirconia single crystals between 1400°C and 1800°C. The mechanical parameters measured have been adjusted to a conventional creep law. The temperature dependence of the strain rate has been fitted to a conventional Arrhenius plot. From the stress exponents and activation energies found, it is concluded that there is a remarkable transition in the mechanical behaviour of this solid solution at T≅1500°C. This behaviour is compared with one found for lower Yttria concentration single crystals, within the full stabilized field.