There is now enough observational information available to show that the interstellar magnetic field in the general neighborhood of the Sun is, on the average, parallel to the plane of the Galaxy, with an average strength somewhere between 10-6 and tO-' gauss. This paper points out certain dynamical requirements for the existence of such a field. The paper is based on the assumption that the intergalactic medium, whatever it may be, exerts pressures on the Galaxy that are small compared to 10-12 dyne/cm'. It can then be shown that the galactic, or interstellar, magnetic field must be to the Galaxy by the weight of the gas threaded by the field and distributed throughout the disk of the Galaxy. It is then shown that the interstellar gas-field system is subject to a universal Rayleigh-Taylor instability of such a nature that the interstellar gas tends to concentrate into pockets suspended in the field. The cause of the instability may he thought of as a hydromagnetic self-attraction in the interstellar gas, which may be ten times larger than the gravitational self-attraction of the gas. It is this hydromagnetic self-attraction which produces the observed tendency of the interstellar gas to be confined in discrete clouds. The calculations and arguments do not restrict the over-all topology or the strength of the galactic field, which apparently must still be determined from observation.