The mechanical behavior of the novel fiber CeraFib75 after various thermal exposures is examined. This fully crystalline mullite fiber was developed to exceed the thermal stability of commercially available oxide fibers. Therefore, heat treatments at temperatures ranging from 1000 degrees C to 1400 degrees C for 25 h were performed and results compared to the well-established Nextel 720 fibers. Mechanical characterization was realized with bundle tensile tests using acoustic emission sensors to determinate the fiber failure distributions. Investigations showed that the initial fiber microstructure of mullite grains with traces of alumina transforms starting at 1200 degrees C. Changes include dissociation of the alumina-rich mullite phase and grain growth. Thus, strength reduction is measured as a result of these microstructure transformations. Remarkably, at 1400 degrees C, fibers become more fragile and Weibull statistics can no longer describe the failure distribution. A relation between the distribution shape and the load redistribution capability of fibers is suggested. This is more pronounced for Nextel720 fibers, which present much bigger grains and retain only 10% of their original strength. However, CeraFib75 fibers are more stable and exhibit a strength retention of 50% at the same conditions, which is attributed to the higher amount of mullite phase.