Using the classical stacking-fault pyramid as an internal reference, the fault planes and stair-rod dislocations in stacking-fault trapezoids and stacking-fault tubes in pseudomorphic ZnSe/GaAs(001) epitaxial layers have been characterized. The acute-obtuse stair-rod dislocation pair in a stacking-fault trapezoid form an extended C dislocation dipole. Stacking faults with a lozenge-shaped cross-section in a stacking-fault tube intersect to give rise to dislocation dipoles of alternating acute-obtuse stair-rod dislocations. The extended dislocation dipoles of the stacking-fault trapezoids and stacking-fault tubes can act as diffusion channels for pipe diffusion of point defects during the degradation process.