Single crystals of electrolytic tough pitch copper and of high purity (99.999%) copper were deformed by bending. The substructure in the as‐bent and annealed crystals was studied by means of sensitive x‐ray techniques. It was found that regardless of purity, crystals of certain orientations in the as‐bent condition do not have sub‐boundaries detectable by the methods used. Polygonization took place only upon prolonged annealing at around 97% of the absolute melting point and, at least in one high purity crystal, not even under those conditions. The sluggishness of the polygonization in copper is experimental evidence for the low rate of climb of extended dislocations predicted by Seeger.