Here we present evidence for gravity wave seeding of ionospheric plasma instabilities. On October 2, 2002 a series of magnetic substorms led to a large-scale traveling ionospheric disturbance (LSTID) that was detected by the Arecibo incoherent scatter radar. Several large oscillations of the F layer occurred, including one that lifted the layer to over 450 km. At least two shorter period oscillations were identified in the plasma contours along the bottomside of the layer. The largest amplitude signal exhibited a downward phase velocity, vertical wavelength, and period (∼50 minutes) consistent with the linear theory of gravity waves. A ∼30-minute period oscillation was amplified considerably when the bottomside was lifted to high altitudes near dawn. This amplification was likely due to a gravitational instability with a growth time of about 15 minutes. We believe that the LSTID created conditions conducive to instability, while the shorter period waves created the seed irregularities.