We address the problem of frontal stability by making use of the geostrophic momentum approximation to the full hydrodynamic field equations. In a previous analysis of this same problem (Moore and Peltier, 1987) that was based upon use of the full hydrostatic primitive equations, we demonstrated that fronts were subject to a cyclone scale mode of baroclinic instability that had not been isolated in any previous stability analysis. Subsequent work (Moore and Peltier, 1988) demonstrated that this new source of instability was completely filtered when the analysis was performed using conventional quasi-geostrophic theory. Not only did quasi-geostrophic theory fail to account for the cyclone scale mode, however, it also rather severely distorted the long wavelength Charney-Eady mode. Our geostrophic momentum analysis of the frontal stability problem reported here shows that this higher level of approximation similarly filters the cyclone scale mode but it does completely correct the phase speed and growth rate errors for the long waves that were introduced by the quasi-geostrophic approximation. In this sense the geostrophic momentum approximation is shown to constitute a useful intermediate form of the dynamical equations. Because it filters the cyclone scale mode, however, it is nevertheless flawed in a rather fundamental way.