The effect of some microstructural heterogeneities on the superplastic behaviour is studied. Grain size distributions are taken into account with a particular attention given to the spatial distribution of the grains through the microstructure. The predictions concern both the macroscopic behaviour and the extent of strain localisation. It is shown that when a superplastic microstructure contains large grains, the parameters which can influence the mechanical behaviour are not only the proportion of large grains or their size but also their degree of agglomeration or even the spatial distribution of these agglomerates.