This paper describes a new modulation technique called DSK (double phase-shift keying) which shows an interesting antimultipath feature over mobile frequency-selective fading environments. The effect of this technique is shown for various multipath channels by detection waveform and theoretical bit error rate (BER) analyses. A result obtained from a laboratory test is also presented. In the latter part of the paper, a generalized form of DSK is described and extensive investigations are made so as to clarify the anti-multipath effect of DSK in view of the diversity concept and to further explore the possibility of a narrowband scheme. A major result shows that this technique can give a kind of diversity effect which is implicit in the arrival of signals via multiple paths with different delays.