The turnover of interstellar gas on ∼ 109 yr timescales argues for the continuous operation of a galactic dynamo. The conductivity of interstellar gas is so high that the dynamo must be "fast"; i.e., the magnetic field must be amplified at a rate nearly independent of the magnetic diffusivity. Yet all the fast dynamos so far known, and all direct numerical simulations of interstellar dynamos, yield magnetic power spectra that peak at the resistive scale, while galactic magnetic fields have substantial power on large scales. In this paper we show that in weakly ionized gas the limiting scale may be the ion-neutral decoupling scale, which, although still small, is many orders of magnitude larger than the resistive scale.