Molybdenum disilicide polycrystals were deformed in constant strain rate tests in compression followed by analyses of the deformed specimens by means of optical microscopy and scanning and transmission electron microscopy. The strain rate sensitivity was measured by stress relaxation tests. Using a low strain rate of 2.5 x 10-7 s-1, it was possible for the first time to achieve plastic flow at low-temperatures down to 495 ^°C and thus in the temperature range of the flow stress anomalies of different slip systems in single crystals. In addition, in situ straining experiments in a high-voltage electron microscope were performed to observe the deformation processes directly. The deformation is controlled by dislocation glide on the 0 1 1<1 0 0> and 1 1 0<1 1 1> slip systems below 1000 ^°C, however, by visco-elastic grain boundary glide and decohesion above this temperature.