Studies of the propagation of 16 kc/s waves from the Rugby transmitter GBR have been made with the final objective of assessing the polarization errors to be expected in taking bearings on lightning flashes at similar frequencies. Measurements were made of the changes of the apparent bearing of the transmitter caused by changes in the amplitude and polarization of the ionospheric waves. A crossed-loop cathoderay direction-finder was used for this work.Polarization errors were largest at distances of about 300 km from the transmitter, where median errors of the order of 10° in daylight and 30° at night were observed. Reasons are advanced for expecting rather smaller errors on a transient signal, such as an atmospheric, but the difference should not be statistically great. Conditions which lead to large errors on a c.w. signal, however, often produce such complex traces with atmospherics that a bearing cannot be read. This is particularly true at night, and common features of night-time observations are missed observations at one or more stations and sets of bearings from which no fix can be derived. A worth-while improvement in fixing accuracy might result from weighting the bearings according to the distance of the flash from each station.The measurements have yielded further information on the reflecting properties of the ionosphere at 16 kc/s. Variations in bearings taken simultaneously at two stations at similar distances from Rugby, but in different directions, indicated that there were significant differences in the propagation along the two paths. It has been concluded that the polarization of the waves reflected by the ionosphere depends on the azimuthal direction of the propagation path, and this may be the explanation of apparent disagreements between the results of previous workers.