The anelastic relaxations observed between 100 and 600 K for a very high purity magnesium are presented. The performed experiences show an internal friction peak P1 placed around 410 K whose activation parameters are determined (v = 1000b3, HP1 = 1 eV). The P1 behaviour is similar to that of the P1 peak observed in 99.9999% Al around 450 K. So, the P1 relaxation in magnesium may be attributed to dislocations glide ruled by jogs climb and vacancies diffusion along the dislocation line.