[en] The Idaho National Engineering Laboratory is actively involved in the development and application of fiber-optic-based sensor technology for use in nuclear reactor research. This paper describes work recently completed to characterize fiber-optic attenuation induced by gamma and neutron radiation in a reactor. The specific goal was to make in-situ attenuation measurements of promising waveguide materials during radiation exposure at elevated temperatures characteristic of reactor environments. Testing was done in a swimming-pool-type reactor generating about one kilowatt thermal power and producing dose rates of about 10₉ n/cm₂/sec fast neutrons (>1 MeV), 10₁₀ n/cm₂/sec thermal neutrons, and 6 x 10₆ rad/h gamma radiation. Samples of state-of-the-art radiation-resistant waveguides were received from five manufacturers and tested simultaneously. One set of waveguides was held at 180₀C temperature and the other set remained at 20₀C. Attenuation was monitored continuously in several spectral bands in the 600 to 1050 nanometer region. Test results are presented