In situ straining experiments on Al-Pd-Mn single quasicrystals in a high-voltage electron microscope between 675 °C and 750 °C showed viscously moving dislocations with segments oriented in preferred directions. First measurements from video recordings indicate that dislocations on parallel traces move at two different velocities. For the first time, macroscopic samples were deformed in compression below 680 °C. Above a transition temperature of about 635 °C for specimens of a fivefold compression axis and a strain rate of 10⁻⁵ s⁻¹, the specimens show a yield point followed by a range of steady state deformation. Below that temperature, they are brittle. However, plastic deformation was achieved down to 555 °C by using a lower strain rate and performing stress relaxation tests before the fracture stress was reached. The samples then show very strong work-hardening. The rapid decrease of the activation volume with decreasing temperature, which is characteristic of the high-temperature range, does not continue at low temperatures. The results are interpreted by dislocation generation and recovery and by the cluster friction mechanism controlling the dislocation mobility.