The characteristics of plastic deformation of an ultrafine-grained (UFG) Al-30. wt.% Zn alloy were investigated using depth-sensing micro- and nanoindentation. Emphasis was placed on the effects of grain boundaries and the unusually high strain rate sensitivity. It is shown that there is a close relationship between enhanced strain rate sensitivity and ductility in this UFG material and this is associated with grain boundary sliding and enhanced diffusion along the Al/Al grain boundaries which appear to be wetted by Zn-rich layers.