A digital modem is proposed which uses a frequency-hopping sixth-order frequency diversity signal, and which incorporates the automatic excision of interference from other HF users. The modem has the property of achieving low error rates at low data rates (75 bits/s), in the presence of severe interference from other HF users, frequency-selective fading and noise. The interference is defined by simple empirical parameters, which are used to determine the theoretical performance of the modem in a combination of the above channel disturbances. An example of experimental results is also presented.