The effect of intersymbol interference resulting from non-matched receiver filtering on the bit error rate (BER) performance of π/4-DQPSK systems recently adopted in the North American and Japanese digital cellular standards, is analyzed in Rayleigh fading. With a Gaussian or a Butterworth (of order N, 2N10) receiver filter, the BER performance is found to degrade by only a small fraction of a decibel from the performance with ideally matched receiver filters. A 4th-order Butterworth receiver filter leads to BER curves which almost coincide with those of the ideally matched filtering condition.