The stability of two‐dimensional convection rolls in a horizontal fluid layer heated from below with rigid boundaries is investigated theoretically for the case when a vertical magnetic field permeates the layer. It is assumed that the magnetic diffusivity is large compared with the thermal and viscous diffusivities of the fluid. Low Prandtl numbers are emphasized in order to make the analysis directly applicable to experiments on convection in mercury and liquid sodium. The theory predicts that the onset of the oscillatory instability of convection rolls is delayed by the effects of the magnetic field and that the parameter range over which steady rolls are stable increases with the magnetic field strength. In the parameter regime investigated, there does not seem to exist a new mode of instability introduced by magnetic forces. It is also found that the magnetic field tends to increase the efficiency of the convective heat transport thereby conpensating in part for the delay in the onset of convection caused by the stabilizing effect on the Lorentz force.