The collisional friction force between two plasma species determines several quantities, such as particle diffusion rates, of central importance in tokamak transport theory. For an impure plasma in a regime of mixed collisionality, as pertains to most present tokamak experiments, electrostatic trapping leads to an unusually strong friction force, associated with slowly moving particles, which cannot be evaluated using the standard techniques of neoclassical kinetic theory. The Pfirsch–Schlu¨ter part of the friction force is computed, and shown to depend logarithmically on parameters characterizing the tokamak equilibrium.