Radio waves with a freuency of 4 MHz transmitted vertically into the polar ionosphere and reflected in the E layer occasionally show very stable oscillations in intensity. This phenomenon, which occurs mainly in the evening sector under quiet conditions, has been studied at Ny-Ălesund, Spitzbergen, by using a 4-MHz transmitter and four spaced receiving antennas. The observed oscillations can be explained as interference of waves reflected from a pair of enhanced ionization regions some 30 km apart and moving with a horizontal velocity of the order of 100 m/s. The low drift velocity is consistent with low magnetic activity.