  We present a novel theory of charge-carrier mobilities in organic molecular
crystals of high purity. Our approach is based on Holstein's original concept
of small-polaron bands but generalized with respect to the inclusion of
nonlocal electron-phonon coupling. We derive an explicit expression for the
mobilities as a function of temperature and, using ab-initio methods to obtain
the material parameters, we demonstrate its predictive power by applying it to
naphthalene. The results show a remarkably good agreement with experiments and
provide new insight into the difference between electron and hole mobilities as
well as their peculiar algebraic and anisotropic temperature dependences.
