We will demonstrate, that in fiber suspensions the stress tensor has a skew-symmetric part, i.e. fiber suspensions are micropolar fluids with non-zero couple stresses. Consequently, a continuum mechanical description of fiber suspensions requires the consideration of the balance of angular momentum in addition to the balance equations of mass, momentum and energy. An antisymmetric stress tensor is found for an isotropic orientation distribution of deformable fibers as well as for an anisotropic distribution of fiber orientations. Mainly, the method of classical Thermodynamics of Irreversible Processes with internal variables is applied. The internal variables are fiber deformation variables and an orientation tensor in the case of a non-isotropic distribution of fiber orientations. In addition to the result on couple stresses, relaxation equations for the bend and twist deformation variables are derived. Finally a mesoscopic background theory is discussed. In this approach a distribution of different fiber orientations and fiber deformations is assumed. The fiber orientation and deformation are additional variables in the domain of field quantities. The usual macroscopic fields of continuum mechanics are averages of mesoscopic fields over the different orientations and deformations. Macroscopic quantities characterizing the orientational order, the alignment tensors, and quantities characterizing fiber deformation, are defined by the help of the distribution function. They are internal variables in the macroscopic theory. The mesoscopic theory also demonstrates, that there is a skew symmetric part of the stress tensor in fiber suspensions.