Evidence is presented that the unusual lightning-generated whistler signals detected very near Neptune are possibly impulsive emissions propagating in the z-mode. Unlike terrestrial whistlers in the plasmasphere, the Neptunian emissions might be propagating in a region where the plasma-to-cyclotron frequency ratio is much less than one. If the nightside plasma density is indeed low, the z-mode will extend down to unusually low frequencies. The observed events may then be interpreted as z-mode instead of the whistler mode. Propagation in the z-mode may explain many of the unusual emission features, including the low frequency limit of 6 kHz and the very large dispersions. Given an atmospheric emission source, this result suggests that Neptune's ionospheric electron density is relatively low. In order for the dispersive events to be observed at Voyager in the z-mode, some locations in the nightside ionosphere are required to have densities as low as about 100 cm-3. A possible magnetospheric source for the emission is also discussed. ¿ American Geophysical Union 1996