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Room temperature tensile tests were performed on AlMg6.5 alloy sheet, with initial strain rates of 6.7×10 ⁴ s ¹, 6.7×10 ³ s ¹ and 6.7×10 ² s ¹, after imposing 70% cold rolling reductions, and annealing at 320°C/3h. The Portevin-Le Chatelier effect is the main mechanism of unstable plastic flow, caused by Mg solute-dislocation interactions. Although, a comprehensive comparison of the energies of activation, relevant for serrated yielding, revealed a wide range of values, calculations based on the used strain rates, retained vacancy concentration and the Mg solute diffusion coefficient at room temperature, indicate that serrations are expected to appear from the start of plastic deformation. In other words, under the performed test conditions, the Mg solute atoms are mobile enough for the majority of dislocations to travel in conjunction with the Mg atmospheres. After deformation at a strain rate of 6.7×10 ² s ¹ the "D" type serration observed, as a series of successive not completely flat stress plateaus, indicates a complex deformation process, affected by strain hardening, consed by dislocation reactions and the propagation of a kind of non-hardening deformation bands.