Observations of very rapid phase advances of up to 8¿ for subionospheric VLF transmissions over a path length of 5745 km are found to be associated with whistlers. We show that the energetic electrons that precipitate in the D region and cause the phase advances interacted with the whistler on its second hop, which is consistent with amplification of whistlers by cyclotron resonance with electrons traveling in the opposite direction. No fast amplitude variations were detected. We propose a simple two-mode model to describe the effects of nighttime D region disturbances on long-distance subionospheric VLF propagation, and we show how this model can account for our observations Helliwell, Katsufrakis, and Trimpi.