We address the problem of multiuser detection in synchronous code-division multiple-access systems in the presence of narrowband external interference. The proposed approach relies on processing the observables in the frequency domain, so as to exploit the narrowband nature of the interferers. The resulting detector represents a generalization of the decorrelating detector to which, in fact, it reduces for the special case that no external interference is considered at the design stage. The performance assessment shows that the newly proposed receiver largely outperforms the decorrelating detector in the presence of narrowband interference, while suffering a negligible loss as no interferer is active