On the basis of a mean-field treatment of semiflexible polymer chains, we analyze the structure of long wormlike polymers sterically confined between two parallel, structureless walls separated by a distance W. With the Onsager excluded-volume interaction between polymer segments, the system displays three phases, uniaxial, biaxial, and condensed, depending on the polymer density and W. We have also determined the induced attractive force acting on the walls, mediated by the Onsager excluded-volume interaction between polymer segments.