Fluctuations of the F-region electron density in the gravity wave period range have been monitored by using the Arecibo incoherent scatter radar. The radar was pointed in the vertical direction, and a transmitted pulse 50 &mgr;s in length was used to provide 1-min time resolution time series of 4- to 30-hour duration at several altitudes in the altitude range of 197.85--485.85 km. Spectral analysis has been performed on 4-hour segments of these time series. Spectral peaks recurring at a nearly constant frequency over the entire altitude range of this experiment were taken to be the ionospheric response due to the passage of an internal gravity wave. Relative phase information at the corresponding frequency can be obtained from the time series data. This response in magnitude and phase was fit to an atmospheric model to yield the parameters of causative internal gravity waves. Two cases are presented and compared to a dispersion relation, generally good agreement being obtained. Discrepancies in the agreement for the imaginary part of the wave number can be attributed to inaccuracies in the model atmosphere, but a disagreement found between the experimental results for the real part of the wave number and the dispersion relation at altitudes above the electron density maximum is of unknown origin.