Various kinds of silica glass and natural and synthetic crystals have been irradiated with fast neutrons at temperatures of ∼250°C and above 300°C. In the glasses, asymmetric paramagnetic resonance lines resulting from the irradiation have been observed with apparentgvalues of 2.0013±0.0006 and 2.0090±0.0007, and half‐widths of ≈1.7 gauss and ≈40 gauss, respectively. Two groups of lines were found in the natural crystals and have been associated with the two lines in the glasses, the two lines in the glasses being the envelope of the lines in the crystals when they are summed over random orientations with respect to the magnetic field. On the basis ofgvalues, absence of common impurity and of hyperfine splitting, and the thermal stability of the lines, it is concluded that the observed resonances in the silica glasses and crystals have their origin in defects in the basic SiO4tetrahedra generated by fast neutrons and primary and secondary knockons. From preliminary optical bleaching data in neutron irradiated Corning silica it is tentatively concluded that the resonance withg=2.0013 is associated with the defect which gives rise to an optical absorption band at 2120 A. Thegvalues of the two lines in the silica glasses suggest that one center is an electron and the other a hole.