Tensile tests were carried out at 473 K with initial strain rate of 10-4 s-1 in samples of a ZK60 alloy (Mg-5.5% Zn-0.5% Zr) processed by different number of passes using Equal-Channel Angular Pressing (ECAP). The measured superplastic elongations ranged from ~930% to a record of ~3050%. The flow behavior was found to vary with the number of passes of ECAP. It is shown that strain-hardening due to grain growth and the evolution of the strain rate sensitivity with the strain determines the flow behavior and final elongations. The results are consistent with theories of plastic flow in tensile testing.