A detailed study was conducted to evaluate the constant stress tensile creep behaviour of a superplastic 3 mol % yttria-stabilized zirconia-20 wt % alumina composite. The comprehensive experimental results indicate that creep deformation may be expressed in the form epsilon proportional to sigma(2.8)(1/(L) over bar)(2) exp(-585 000/8.3T), where epsilon is the steady-state creep rate, sigma is the imposed stress, (L) over bar is the linear intercept grain size and Tis the absolute temperature. Microstructural observations revealed that there is very little dislocation activity, or change in grain size or shape. A detailed analysis was conducted to evaluate the possible rate-controlling mechanisms in terms of the experimentally determined mechanical properties and the microstructural observations. Based on the maintenance of an equiaxed microstructure and the strong grain size and stress dependence, it is concluded that creep occurs by a grain-boundary sliding/grain rearrangement process.