A commercial polycarbosilane, thermolysis product of polysilastyrene, was spun into fibres. The self-curing character of the polymer permitted both oxidative and thermal curing of the precursor fibres. Pyrolysis of the latter produced ceramic fibres in high yields, which were fully characterised. Optimum tensile strength was attained after heat treatment at 1100°C. Oxidative reactions and crystallisation caused strength degradation above this temperature.