With the restrictions and guidance provided by nonequilibrium thermodynamics in mind, we develop a thermodynamically admissible single-segment reptation model that unifies all the effects identified as relevant to the flow behavior of polymer melts on the reptation time scale. Starting from a formulation of a reptation model without independent alignment, we incorporate Marrucci＇s recent ideas of convective constraint release and anisotropic tube cross sections, and in particular, we provide consistent time-evolution equations for the anisotropic tube cross section in flow. Important features such as irreversibility in double-step shear strain experiments with flow reversal and a non-decaying shear stress at high shear rates are appropriately seized by the proposed model. The ratio of normal-stress differences is determined in terms of the change of the mean-square curvature of the tube cross section in shear flow. A model simplification ideal for efficient computer simulations and for matching the experimentally observed linear viscoelastic behavior is proposed. (C) 2000 Elsevier Science B.V. All rights reserved.