In the present study the superplastic behavior of Al-6%Mg-0.5%Cu and Al-8%Mg0.5%Cu in a coarse grain size condition has been studied. The alloys are melted in an electrical furnace under argon atmosphere. The ingots (25 mm thick) are homogenized at 400 degrees C during 72 h and then rolled at 430 degrees C to a thickness of 5 mm. The mean grain size after rolling is 55 mu m for the 6%Mg alloy and 61 mu m for the 8%Mg alloy. Tensile test specimens are machined from the rolled plate in the rolling direction. Strain-rate-change tests at temperatures between 300 and 450 degrees C and strain rates between 1x10(-4) and 1x10(-1) s(-1) are carried out to determine the strain rate sensitivity of the flow stress. Finally, elongation to failure tests are conducted at temperatures and strain rates where the alloys show a high strain rate sensitivity. Elongations higher than 390 % are obtained for the 8%Mg alloy. It is observed that the grip regions of the deformed samples show coarser grains than the regions near to the fracture surface. This means that grain refinement takes place during deformation, suggesting that the principal deformation mechanism is dislocation creep.