Single crystals of commercially pure (2S) aluminum were stressed in push‐pull and the development of surface fatigue cracks observed continuously with a specially mounted 300× optical microscope. Time‐lapse photographic sequences were used to measure the propagation rate and to determine the relation between surface slip markings and crack propagation mechanisms. Cross‐sectional views of the internal disposition of the crack were obtained by sectioning in a microtome. Back reflection x‐ray photographs were used to gather evidence concerning the deformation substructure near the fatigue crack. The data thus assembled allow the mechanism of fatigue crack propagation to be described in terms of surface slip bands, the stress concentration at the edges of these bands, and subgrain formation which occurs in the vicinity of the crack tip. The linear relationship between the logarithm of the instantaneous crack length and the number of stress cycles was confirmed and the proportionality between the crack length and the plastic zone immediately ahead of the tip was demonstrated.