Fe–43at%Al single crystals were deformed in compression between room temperature and 700 ^°C. The strain rate sensitivity of the flow stress was measured by stress relaxation tests. The dislocation structure of the deformed samples was studied in a high-voltage transmission electron microscope. In addition, micro-tensile specimens were deformed in situ in this microscope to directly observe the dynamic behaviour of the dislocations. The specimens show a flow stress anomaly accompanied with an ‘inverse’ dependence of the strain rate sensitivity on the strain rate. Both the in situ experiments and the transmission electron micrographs of macroscopically deformed samples indicate the occurrence of climb at temperatures in and above the anomaly range. In the in situ experiments, viscously moving dislocations are straight and crystallographically oriented. The phenomena related to the flow stress anomaly are interpreted by the decomposition of the cores of dislocations with (111) Burgers vectors.