Studies are presented here on reactor-irradiated alpha aluminum-oxide single crystals. These are a continuation of the use of long-wavelength neutron transmission for determining the concentration and types of defects produced in solids by high-energy particle irradiation. The material exhibited crystallographic stability to fast-neutron irradiation at temperatures <40°C, and the results indicate a total number of defects approximately 40 times less than that predicted by current theories. Correlation with macroscopic density changes is good. Examination of the wavelength dependence of the neutron scattering indicated that the damage may be partly Al-O vacancy pairs at room temperature. Annealing of the material produced no decrease in the concentration of defects from room temperature to 400°C, a steady decrease from 400°C to 1250°C, and nonuniform changes in neutron scattering and visually observable optical coloring beyond 1250°C. Annealing at a temperature of 1800°C did not remove the coloring, although the density returned to its pre-irradiation value.