Direct observation in the scanning electron microscope of grain-boundary sliding (GBS) in Pb-62%Sn eutectic alloy, superplastically deformed in shear, showed non-uniformity of GBS. Such non-homogeneity of GBS reveals itself as sliding of large grain blocks with dimensions of tens of grain size and sliding of grain groups with dimensions of a few (four to eight) grain size. Sliding of large blocks of grains is a result of the sliding of grains as an entity along grain boundaries of former dendritic boundaries. The sliding of grain groups is due to the cooperative manner of GBS. Experimentally observed size of the grain groups can be explained from the view point of cooperative GBS, caused by glide of cellular dislocations.