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In this paper a useful and mathematically rigorous method is presented for exactly evaluating the error-probability performance of a binary differentially coherent phase-shift-keyed (DPSK) system in the presence of both thermal noise and intersymbol interference. The channel filter and the receiver input signal-to-noise ratio are assumed to be given. This method suggests, first, to calculate the probability of error induced by both the thermal noise and the large interfering samples whose normalized amplitudes with respect to noise standard deviation are greater than unity, and then, to add the correction terms due to the presence of the other intersymbol interference samples. If the normalized interference amplitudes with respect to the noise standard deviation are less than unity (although the total interference power may still be greater than total noise power), then this method simply becomes that which first evaluates the error probability induced by thermal noise alone and then adds the correction terms due to the presence of intersymbol interference. For numerical computations, various recurrence relations are developed so that the probability of error can be easily obtained on a digital computer. These relations have been utilized in an example to calculate the performance of certain practical PSK and DPSK systems.