An analysis of the experimental evidence in Part I of this paper clearly indicates that for the case of copper crystals, striations in low‐amplitude fatigue are suppressed by strong slip on intersecting slip planes, including the cross slip system, but are formed by interactions between dislocations on the primary glide system with dislocations on another glide system sharing the same slip plane. This observation is in obvious contradiction toany theory based on cross slip of the primary dislocations alone, since cross slip combined with glide and climb allows every conceivable rearrangement of the dislocations but cannot change the Burgers vectors. A model for the mechanism of striation formation is therefore developed which takes into account that dislocations with at least two different Burgers vectors on the primary slip plane cooperate, according to the experimental evidence. It is proposed that striations consist of sequences of ``cells,'' forming pairs of dense dislocation networks parallel to the primary slip plane, separated by narrow channels almost free of dislocations and debris. The networks consist of almost equal surplusses of dislocations with the three ½ ⟨110⟩ Burgers vectors in the active {111} slip plane, representing more or less well‐developed twist boundaries with a greater or smaller neutral filling of other dislocations and debris. Once striations are formed, glide becomes concentrated in them and hardening proceeds at a much slower rate or comes to saturation. Extrusions and intrusions can arise in at least three different ways and it is believed that these mechanisms act singly or in combination, depending on conditions. It is shown that the model is in good accord with previous evidence. In particular, dislocation structures associated with striations in fatigued copper, observed by Laufer and Roberts, are very close to those described. A few simple numerical relationships are derived from the model which could be employed to test the theory presented. Much of the theory is believed to be applicable also to dislocation behavior in unidirectional work hardening of fcc single crystals.