We have extended our study of shear flow instabilities in nematics, aligned perpendicular to the velocity and velocity gradient, to the plane Poiseuille flow case. We recover the homogeneous as well as the roll instability modes. The non-uniform anisotropic stress tensor gives rise to body forces which deflect the flow lines and couple with the distortions. Novel structures are met due to the existence of distortions near both the upper and lower plates. A new variable-wavelenght linear mode is found.