The room temperature brittleness of NiAl constitutes a major problem for technical applications. In order to investigate the micromechanisms of fracture in NiAl, we have carried out in situ tensile straining experiments on stoichiometric NiAl single crystals in a high-voltage electron microscope. According to our observations, crack propagation always involves dislocation activity around the crack tip, even in the hard orientation at room temperature. The Burgers vectors and the typical arrangements of the dislocations, as well as the extension of the corresponding plastic zone vary with the loading direction and the orientation of the microcrack versus potential glide systems. We observe that local concentrations of slip leads to irregular deviation of the cleavage plane from the 1 1 0 facets one usually observes at the macroscopic level. The results of our experiments help to understand why the mode I fracture toughness of NiAl is significantly larger for <100> loading directions than for non-<100> directions.