A new nonlinear mode of a density front is presented and we show that long, finite deviations from a steady state of the free streamlines of the coupled front of zero potential vorticity lead to oscillations of these streamlines about their initial locations. The amplitude and frequency of such oscillations are computed and it is found that the frequency depends on the initial displacement and is always higher than the inertial frequency. The results found for the free streamline behavior are also applicable to the coastal front in which the potential vorticity is zero and the along-stream velocity vanishes at the coast.