Small fluctuations in the critial frequency of the F region plasma have been measured at Arecibo with frequency resolution better than 1 kHz and time resolution of 1 s. For periods in the 10- to 20-min range, f0F2 fluctuations fall off with fluctuation frequency (f) as f-k, with k?4.5. While no oscillation is observed in the period range 4--8 min, significant oscillations with amplitude of Δf0F2≈500 Hz are observed with a period of &tgr;≈3 min, corresponding to a fractional change in number density of 1--2 parts in 104. Several possible sources of these fluctuations are explored acoustic waves in the F region, hydromagnetic or Alfven-type oscillations, and small and large scale electric field fluctuations. Coordinated experiments employing incoherent scatter, magnetograms, and preferably an HF-CW doppler sounder would be able to resolve the origin and nature of these intriguing waves observed for the first time in f0F2 fluctuations.