This in vivo study evaluated the fracture resistance of bovine teeth with prefabricated carbon fiber posts. Fourteen bovine teeth having similar lengths and dimensions were mounted in an acrylic resin block having a simulated periodontal ligament. The post space was prepared using two calibrated drills that provided an 8.5 mm post length. The prefabricated carbon fiber post was luted with a resin luting agent, and the core was made using the system's autopolymerizing resin core material. A crown was luted to each prepared tooth. Each test specimen was intermittently loaded (250N) at an angulation of 45 degrees to the long axis of the tooth at a frequency of 2 loads per second. Four of the roots had an incomplete longitudinal fracture after loading. The results of this study were compared to a previous study by the authors that had been conducted under similar conditions with prefabricated parallel-sided posts (Para-Post) and tapered, individually cast posts. The failure rates of the two types of posts from the previous study were significantly higher (Logrand test; P.02) than those of the carbon fiber posts.