This paper describes the first experimental determination of the migration temperature of freely diffusing self-interstitials in magnesium. This observation was made possible by analysis of the elastic modulus and damping variations induced by the pinning of the dislocations by self-interstitials. A marked pinning stage is found between 9 K and 14 K, which is definite evidence for the long-range diffusion of the self-interstitial at these temperatures. Prior data are discussed and reinterpreted in the light of this finding.