The performance of a differential phase-shift-keying (DPSK) direct-sequence (DS) spread-spectrum receiver using decision-feedback (DF) filters in the presence of narrowband interference and multipath is considered. First, the general analytic expressions for the optimum tap weights of the DF filters and minimum mean square errors are obtained. Next, the authors calculate the output signal-to-noise ratio (SNR) of the system having the optimum tap weights. The authors also consider the bit-error-rate (BER) by computer simulation. Finally, the adaptive DF filters using a recursive least square (RLS) algorithm are considered. It is found that the two-sided DF filter is superior for suppressing both interference and multipath in the spread-spectrum system. The authors also find that the adaptive DF filter using the RLS algorithm has rapid convergence.