The thermal conductivity, λ, of three samples of oriented fibrous carbon insulation of possible interest to fusion reactors was measured from 300 to 1300°K in a radial heat-flow apparatus. Samples of 0.18 g/cm3 density were prepared by a vacuum filtration process from carbon fibers and powdered phenolic resin and were characterized after carbonization. The λ of these low-density composites depended on both the heat treatment temperature and the fiber orientation. For samples heat treated at 1575°K, the room-temperature λ perpendicular to the planes of fibers was ∼0.5 mW/(cm °K) and was three times as high in the direction parallel to the planes. At 1000°K, the λ in both directions had doubled, primarily because of the positive dλ/dT of the amorphous carbon fibers. Material heat treated at 2775°K had a significantly higher room-temperature λ and a negative dλ/dT, indicating that a slight degree of ordering or graphiti-zation had occurred in the fibers during heat treatment. At high temperatures, the λ of all three samples increased markedly because of radiative heat transport.