Experiments are described in which growing fatigue cracks with simple and well reproducible crack geometries can be observed during cyclic loading inside a SEM. Copper and Fe-2.4%Si single crystals of appropriate shape and orientation were examined. For the investigation of the slip processes at the apex of the crack tip a commercially available tensile stage was modified to allow for cyclic deformation. It was found that the same mechanism for ductile crack propagation is operative in copper and in Fe-2.4%Si. In these crystals various predictions of the alternate slip model were verified. In both materials the alternate activation of the slip systems can be observed directly. In the Fe-2.4%Si single crystals with their larger variety of slip systems an orientation can be chosen, which further simplifies the stress-strain distribution near the specimen surface. In these crystals even the motion of the crack tip due to the alternating slip—the very mechanism of crack advance—can be observed directly.