SUMMARYAt elevated temperatures, electron irradiation damage in silicon and germanium forms faulted defects on {113} which are elongated in. During the present work on silicon, these {113} faults were observed to unfault, apparently by glide, to give elongated interstitial loops withb=a/2. Since the tetrahedral interstitial sites between adjacent {113}s form a full {113}, in both number and distribution, it was concluded that the atoms were accommodated in these positions.A fault of this type comprises interstitial pairs, the atoms of which are on adjacent tetrahedral sitesa/4apart, and it is proposed that these already exist, or are formed, when a fault nucleates. Two‘di‐interstitials’ then close‐pack along one of the threes normal to theiraxis, thus nucleating a {112} loop. This is unable to grow in any direction other than, because its habit plane is occupied by matrix atoms, so the loops become extended. The defect grows in width by nucleating further loops on {112} planes, extended parallel to the first, and the‘array’ forms an elongated {113} defect, on a plane 10° from the original {112}. This gives the observed faulted {113} loops, elongated in, wi