Measurements of fiber strength utilization (FSU) in unidirectional carbon/carbon (C/C) composites as a function of heat-treatment tempcrature (HTT) have been extended beyond the original group of DuPont pitch-based E-series fibers to include additional pitch- and PAN-based fibers. Fibers and composites were characterized by tensile strength, optical microscopy, SEM, fiber preferred orientation, and a single-fiber composite (SFC) fragmentation test to provide a relative measure of fiber-matrix interfacial shear strength (IFSS). Results show that fracture behavior and FSU are dominated by the degree of fiber-matrix bonding, as inferred from microscopic observations and measurements of IFSS. In the very high modulus pitch-based fibers, the behavior of the F,130 is strikingly different from that of the Amoco and Nippon Oil fibers, in that it retains good bond strength and high FSU even with HT to 2400 °C, in contrast to the other very high modulus pitch-based fibers that are already weakly bonded at the lowest HTT of 1100 °C. All PAN-based fibers and lower modulus pitch fibers are characterized by strong bonding, brittle fracture, and low FSU for the 1100 °C HTT. Subsequent heat treatment of these composites to 2150 and 2400 °C, in most cases, results in significant recovery of FSU, suggesting an optimum IFSS for each composite. It is suggested that the difference in bonding between the pitch-based E-series and P-series may be related to the similarity in fine structure between the E-fibers and high-modulus PAN-based fibers.