Copper single crystals of various orientations have been plastically deformed in alternating torsion, and observations have been made of the appearance of fine slip traces in the initial stage of deformation, the longitudinal elongation, and the transformation of the circular cross section into oval-tetragonal, cruciform, triangular, elliptical, and other sections. Analysis of the slip traces has shown that, with a symmetrical arrangement of the octahedral slip planes with, respect to the axis of torsion, not all of the planes are active, but only a certain number sufficient to convert the elastic deformations into plastic slip. The axial elongation and the nature of the change in shape of the cross section depend on the number and orientation of the active slip planes.