Macroscopic compression tests and in situ straining experiments in a high-voltage electron microscope were performed on NiAl-0.2at.% Ta at room temperature and at elevated temperatures. At room temperature in soft orientations, dislocations of a[100] Burgers vectors bow out between jogs. In contrast to pure NiAl, the dislocations move in a viscous way between the pinned configurations. At 475 degrees C in a hard orientation, dislocations with a[110] Burgers vectors move in a viscous way in configurations strongly depending on the respective slip plane. Preferred orientations of dislocations are of mixed character, most pronounced as very straight dislocations oriented along [111] directions on 110 planes. These configurations cannot be explained on the basis of the existing atomistic theories. The flow stress is interpreted in terms of the back stress of the dislocations bowing out between jogs at room temperature, the statistical theory of solid solution hardening, and the formation of atmospheres containing Ta atoms at elevated temperatures.