By applying a new technique [1-2] which provides a mesoscopic coordinate system inscribed on the surface of a tensile specimen, with 371 mum gage length for a Zn-20.2%Al-1.8%Cu superplastic alloy deformed at room temperature it is possible to show that: Deformation of the sample it is homogeneous at macroscopic level, but inhomogeneous at mesoscopical level. The inhomogeneity is ascribed to the sliding of grain blocks. For 28.5% of deformation the distribution function for the block sizes is described by: N(x) = 1.37 x(3)exp(-3x/12.2 mum), where, N(x) is the number of blocks of size x, inside an area of about 172 X 244 (mum)(2).