The purpose of this paper is to examine the effects of a horizontal magnetic field and of horizontal transport of plasma on the vertical electron density profile in the Martian ionosphere. A two-dimensional magnetohydrodynamic model for the steady state Martian ionosphere is presented. The model takes into account the basic reactions involving O+, O+2, and CO+2 ions and vertical plasma transport as well as horizontal plasma transport along the hypothesized magnetic field lines. The good agreement between model predictions and experimental electron density radio occultation profiles supports the hypothesis of an intrinsic magnetic field of Mars. ¿American Geophysical Union 1995