The paper investigates numerically the effects of a magnetic field upon natural convection of electrically conducting fluids contained in differentially heated square enclosures filled with fluid-saturated porous media. The magnetic field is induced by two electric currents flowing parallel to the vertical walls of the enclosures. The Darcy flow model is used to describe the fluid flow inside the fluid-saturated porous media. It is shown that the magnetic field depends only on the distance to the vertical electric currents and, based on the physical model, is demonstrated that the overall effect of the induced magnetic field is to decrease the intensity of natural convection inside the enclosure; therefore, the overall heat transfer crossing the enclosure also decreases. Results are obtained for some particular combinations of the dimensionless governing parameters, and especially for some particular values of the Hartmann number, showing that a reduction in natural convection occurs, when the intensity of the magnetic field increases. Fluid flow structure and thermal field are analyzed through the streamlines and the isotherms. For the physical configuration under analysis, it is shown the effect of the induced magnetic field is always a reduction of the natural convection taking place inside the enclosure, and the consequent reduction of the overall heat transfer crossing it. (C) 2012 Elsevier Ltd. All rights reserved.