Formulas are presented for the computation of error probability of OQPSK and MSK with narrow-band filters in transmitter and receiver and various detector filters (matched filter, sampling detector, rectangular filter and half-rectangular filter). Numerical results are computed for the case when the filters in transmitter and receiver are identical Butterworth filters. The error probability is presented as a function of signal/noise ratio or normalised filter bandwidth with the filter order as a parameter. The conclusion is that with narrowband filters, MSK with a sampling detector or half-rectangular detector filter and OQPSK with a sampling detector are the best systems. Although MSK is slightly better than OQPSK, the difference in performance is insignificant for the range of bandwidths of practical interest.