Studies dating back more than 15 years have presented evidence that atmospheric gravity waves play a role in initiating nighttime equatorial F region instabilities. This paper analyzes a spectacular spread F event that for the first time demonstrates a layering which, we argue, is controlled by a gravity wave effect. The 50-km vertical wavelength of a gravity wave which we have found is related theoretically to a plasma layering irregularity that originated at low altitudes and then was convected, intact, to higher altitudes. Gravity waves also seem to have determined bottomside intermediate scale undulations, although this fact is not as clear in the data. The neutral wind dynamo effect yields wave number conditions on the gravity wave ability to modulate the Rayleigh-Taylor instability process. Finally, after evaluating the gravity wave dispersion relation and spatial resonance conditions, we estimate the properties of the seeding wave. ¿American Geophysical Union 1990