A numerical study is carried out of the magnetic field effects on the coherent structures and the associated heat transfer in a turbulent channel flow with constant temperature at the bottom (cold) and top (hot) walls. Results from direct numerical simulations are conditionally sampled in order to extract the dominant coherent structures in the near-wall region for flows with and without a uniform external magnetic field in the wall-normal direction. The Reynolds number based on the bulk velocity and the wall distance is 5600, while only a representative small Stuart number of 0.01 is explored. Two fluids with Prandtl numbers of 0.01 and 0.71 are studied. It is shown that the conditionally averaged quasi-streamwise vortices are modified by the magnetic field with their size being increased and their strength decreased. The underlying organized fluid motions are damped by the Lorentz force and the turbulent heat transfer related to the action of quasi-streamwise vortices is decreased by the magnetic field. For the higher Prandtl number fluid, a similarity between the coherent temperature and the coherent streamwise velocity fluctuations is observed for both types of flow. This is diminished for the lower Prandtl number fluid, especially in the magnetohydrodynamic flow, inhibiting the intrusion of cold (hot) fluid from the cold (hot) wall towards the central region. (C) 2011 Elsevier Ltd. All rights reserved.