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The nonlinear minimum mean-squared error (MMSE) decision-feedback multiuser receiver for direct-sequence code-division multiple-access (DS-CDMA) systems is known to offer a large performance improvement relative to linear MMSE-based detection. Such an improvement is made possible through the feedback filter which compensates for the effects of both intersymbol and multiuser interference. Motivated by the potential gain of these nonlinear multiuser detectors, and the severe tracking problems of the standard MMSE adaptive receiver, we investigate the performance of a modified 2-stage MMSE decision-feedback multiuser detector in frequency-selective fast-fading channels. Considering a multipath fast-fading channel, the modified nonlinear adaptive receiver is shown to offer much higher gain than existing modified (precombining) linear MMSE receivers. Our results show that the modified nonlinear receiver is more resilient to signal interference than the precombining LMMSE, especially in heavily loaded systems.