A variety of SiC-Based fibers were characterized by measuring their length, density, and tensile strength in the unirradiated, thermal annealed, and irradiated conditions. The irradiation was conducted in the EBR-II to a dose of 43 dpa-SiC (185 EFPD) at a nominal irradiation temperature of 1000°C. The annealed specimens were held at 1010°C for 165 days to approximately duplicate the thermal exposure of the irradiated specimens. In general, the results of this study indicate the fibers that perform best in an irradiation environment are those that approach stoichiometric and crystalline SiC. Hi-Nicalon exhibited negligible densification, accompanied by an increase in tensile strength after irradiation. Nicalon CG possessed a higher tensile strength than Hi-Nicalon in the unirradiated condition, but was significantly weakened in the annealed and irradiated conditions. In addition, Nicalon CG exhibited unacceptable irradiation-induced shrinkage. While the irradiation stability of Hi-Nicalon was promising, other fibers with compositions closer to stoichiometric SiC may perform even better. This potential was suggested by the MER99 fiber, which displayed excellent dimensional stability. The principal drawback for the fully crystalline and stoichiometric fibers such as MER99 and Crystalline SiC is their low strength and flexibility caused by high flaw concentrations.