A model SiC-fibre has been prepared from a polycarbosilane precursor by means of an irradiation oxygen-free curing process. The chemical composition remains unchanged after heat treatments under an inert atmosphere for pyrolysis temperatures of 1600 degrees C. At this temperature, the fibre consists of SiC nanocrystals (mean size 6-10 nm) and free carbon. However, a slow grain growth takes place as the temperature is increased. The fibre retains a high strength at room and high temperatures up to temperatures of 1600 degrees C when the pyrolysis has been performed under nitrogen. The electrical conductivity was studied as a function of the pyrolysis temperature T-p : For 1100 less than or equal to T-p less than or equal to 1200 degrees C, the conductivity increases by several orders of magnitude due to the reorganization of the free carbon phase at the SiC grain boundaries. Oxidation kinetics of the filaments remain parabolic from 1000-1400 degrees C.