Differentially coherent phase shift keying (DPSK) is commonly used to provide data communication on frequency dispersive channels. To obtain higher data rates one can increase the number of phases or employ combined amplitude-phase modulation. In this paper we compare the performance of multilevel DPSK with the performance of Type I amplitude-phase modulation on a frequency dispersive channel with additive noise. For the Type I system differential modulation is used to amplitude encode a single bit, and ordinary DPSK is used to encode up to four phase bits. Curves of bit error probability versus the effective signal-signal-to-noise ratio of the channel are presented for both the amplitude and phase encoded bits. From these curves the appropriate amount of amplitude modulation to be employed in a high rate Type I system can be obtained. Performance comparisons between multilevel DPSK and Type I modulation are then presented under conditions of both equal average and peak transmitter power.