The etch-pit technique has been used to study dislocations and substructures in copper single crystals deformed in primary, and into secondary creep at 550 °C at a CRSS of 250 g.mm−2. Etch-pit structures in crept specimens were observed parallel to the primary (111) plane and also the (111) plane to reveal "forest dislocations" and primary and other dislocations. In the initial stage of primary creep, the dislocations formed well-defined cells. Subboundaries began to form later on in primary creep. When a steady-state creep ensued, a well-developed subboundary structure was evident, with a random distribution of dislocations within the subgrains. The mean subgrain diameter was approximately twice that of the original cells and the dislocation density decreased by a factor of 2 during primary creep. Since the general form of the dislocation structure changes during primary creep, it is concluded that the decreasing strain rate cannot be ascribed to any one particular mechanism. However, the high initial strain rate probably results from the escape of dislocations from the cell walls in the initial type of structure.