A cleavage crack can generate plastic deformation in a crystal only if it is moving sufficiently slowly or can be brought to rest. This can be demonstrated very strikingly by the cleavage of rock salt or lithium fluoride crystals at a low rate of loading. The cleavage crack propagates discontinuously under these conditions, and thus provides a range of velocities at which deformation might occur. It is found that intense plastic deformation occurs only in narrow zones around the positions where the crack stops; elsewhere the crack runs freely producing a perfect cleavage. It is possible to detect and at the same time estimate the magnitude of this deformation by studying the interference pattern produced by light reflected from the two surfaces of the crack. Observations of this kind allow the deformation to be interpreted as the production of arrays of loops of dislocation by the slowly moving crack. The interference technique is also valuable for displaying the presence of the crack so that its mode of propagation can be studied. A number of observations of the opening and closing of cracks suggest that a study of this kind using a system of reversed stressing might have an important application to the problems of the propagation of fatigue cracks.