The key objectives of this research project were (i) to synthesize fibers from polymer derived silicon carbonitride, (ii) to measure and understand the chemical and structural stability at ultrahigh temperatures, and (iii) to measure the mechanical properties of the fibers. All have been realized. The synthesis of the fibers required innovative modification of a commercial precursor; a U.S. patent for this novel process has been filed. The fibers are shown to have excellent mechanical properties and to be stable up to 1350 degrees C without a measurable change in their nanostructure (which is essentially amorphous). They are thermally stable (against decomposition) up to 1400 degrees C. Interestingly the fibers are a composite of an oxide phase (zirconia) dispersed in the form of nanoscale particles in the polymer derived silicon carbonitride matrix. This novel discovery of oxide/non-oxide polymer-derived-ceramics (PDCs) is leading to new insights into the nanostructure and properties of these scientifically exciting materials. The PDCs are envisioned to emerge as technological important ultrahigh temperature materials for air and space applications.