The results are presented for a 13-month experimental investigation of anomalous propagation on two overland transhorizon paths at 185. 75 MHz. The anomalous signals, 15 to 42 dB above the long-term median and persisting 1 to 6 hours, are described and their possible modes of propagation are reviewed. Surface search radar was found to be a most promising method for monitoring the surface refractivity conditions conducive to high fields. The extrapolation of the experimental results to other regions is discussed and re commendations for further study are included.