This paper describes methods for the serial transmission of data which have been developed and subsequently tested in practice during the course of experiments carried out over a 1000 km h.f. radio link. The methods include the use of adaptive equalization to combat the severe effects of channel-induced intersymbol interference and techniques for securing carrier and bit synchronization in the presence of intersymbol interference. In the system described, a probe pulse is imbedded into the data stream and use is made of the fact that the shortest signal path arising from multipath ionospheric reflections in the h.f. channel is on average the most stable of the multipaths. The presence of the probe signal reduces the data rate by 25%, but enables synchronization to be ensured in the presence of multipath and, furthermore, it enables the channel to be adaptively monitored for equalization purposes. In addition, it can be used to overcome system failure which is inherent in the use of decision-directed equalization operating in the presence of high levels of noise and multipath.