The mechanical properties of commercially available SiC-based ceramic fibres were measured in the temperature range from 400-1300 degrees C. The measurements were performed in air and in inert gas atmospheres, respectively. The Nicalon(TM) and Tyranno(TM) fibres were tested as filament bundles and the decrease in strength occurring at temperatures above 600 degrees C was found in both atmospheres. To obtain a well-defined gauge length at the testing temperature, a furnace with very steep temperature gradients at both ends was built. To eliminate grip-induced damage in the heating zone the fibre bundles were fixed outside the furnace with cold grip units. These grips guaranteed the uniformity of load distribution imposed on to each of the individual filaments in the fibre bundle. A significant shrinkage of the fibres occurring during the creep test performed under low loads indicates a change in the microstructure of the fibres at high temperatures.