A magnetohydrodynamical result is deduced, which could contribute to our understanding of spiral and ring structures in galaxies. The usual expressions for the continuity, momentum and induction equations are adopted for the gas of a galaxy, and the following simplifying hypotesis are made : a) Steady state conditions, b) Axisymmetry, c) A velocity field given by (π=0, θ=θ(r), Z=0) for the interstellar gas (where π,θ and Z are the radial, azimuthal and vertical to the galactic plane components and r is the distance from the galactic center). Then, the direction of magnetic field must be azimuthal and the plasma distribution is compatible with ring structures.