Lithium niobate plates intended for application as sensors for acoustical monitoring of nuclear reactors were subjected to thermal-neutron irradiation near room temperature. By 8 x 1019n/ cm2,  a crystal had become highly disordered, as shown by the back-reflection Laue picture, by loss of piezoelectric response, and by loss of optical birefringence. This transformation appears to be a metamictization. Changes in optical absorption, tritium loss, postirradiation lithium migration, surface crazing, and effects of postirradiation storage at room temperature and of heating to 140°C are also observed.