Defects in annealed P+ion-implanted silicon (ptype), implanted to below the critical dose to form a continuous “amorphous” layer, were found to be faulted hexagonal Frank loops (∼1016/cm3, 200 Å in diameter) on the four {111} planes. A few unfaulted loops and linear defects (∼1013/cm3, ∼800 Å in length) along the ⟨110⟩ directions were also present. The loops showed contrast effects indicative of solute segregation within the loop. The displacement vectors are then of the typea/x[111] withxslightly greater than three. The loops were all interstitial suggesting that they form from the conversion and growth of small interstitial clusters formed during implantation. Segregation of dopants to these interstitial clusters could account for the poor electrical activity of phosphorus in foils implanted to below the critical dose and annealed in the 500–700° range. Studies ofn-type (P and Sb doped) foils show that defect morphology varies with foil type (n-or p-) and dopant species present prior to implantation. Evidence that the linear defects are interstitial type, that they anneal from the end near the free surface and that boron is necessary for their formation is also presented.