Electron spin resonance has been observed inn‐type silicon irradiated with 0.5‐Mev electrons. The particular resonance lines discussed here appear only in pulled crystals which contain about 1018oxygen atoms per cm3. The lines do not appear in floating zone crystals (<1017oxygen per cm3). The pattern is anisotropic with respect to the field orientation and can be fitted with agtensor with componentsg[100]=2.0029,g[110]=2.0019,g[11¯0]=2.0096.Hyperfine structure due to interactions with Si29nuclei is also observed. The hyperfine structure tensor exhibits a [111] symmetry indicating that the electron is not centered on the silicon nucleus with which it is interacting. A model is suggested, consistent with the observed symmetries, according to which vacancies produced during irradiation diffuse to the distorted regions around oxygen atoms.