A Zn-22% Al eutectoid alloy was processed by Equal-Channel Angular Pressing (ECAP) to produce an ultrafine grain size and then pulled in tension at elevated temperatures to evaluate the role of internal cavitation under superplastic conditions. Tensile testing yielded a highest elongation of 2,230% at a strain rate of 1.0 x 10(-2) s(-1) at 473 K representing high strain rate superplasticity. Quantitative cavity measurements were taken to investigate the significance of the internal cavities formed during superplastic deformation. The results demonstrate that cavity nucleation occurs continuously throughout superplastic flow, and there is a transition in the cavity growth mechanism from superplastic diffusion growth at the smaller cavity sizes to plasticity-controlled growth at the larger sizes.