The frequency spectrum of a CW HF radio signal transmitted from an aircraft and propagated over a 1‐hop path inside the polar cap was recorded for about 50 nighttime hr during August 1967. Two different general types of disturbance were observed in these records: (a) a diffuse spectral broadening which was always present to some extent, even under quiet conditions, but which on some occasions was increased by as much as a factor of 2; and (b) discrete frequency anomalies, which were recorded at a rate of about two per hour, and which generally appeared above the carrier frequency (about 9 MHz) and decreased in frequency with a median slope of —2.2 Hz min−1. These discrete anomalies can be interpreted most satisfactorily in terms of Doppler‐shifted reflections from moving ionospheric irregularities. On the basis of this interpretation, the observations are taken to show that irregularity velocities are often in the range from 0.5 to 1.0 km sec−1, with a preferred direction of motion roughly toward the quadrant from geographic E to S and centered on geomagnetic E. However, the measurements were relatively insensitive to geomagnetic N‐S movements and to low