Listeners were required to detect mistuning imposed on the center (“target”) component of a 200-ms complex consisting of the first seven harmonics of a 500-Hz fundamental. In the standard interval of each 2IFC trial, all components were frequency modulated in-phase by a 5-Hz sinusoid. In the signal interval the frequency modulation of the target component was inverted in-phase, thereby introducing a mistuning proportional to the depth of FM. In a similar experiment, using monaural presentation, Carlyon [J. Acoust. Soc. Am. 95, 2622–2630 (1994)] reported a substantial elevation of thresholds in the presence of an unmodulated asynchronous interferer with frequency identical to the mean frequency of the target. This was attributed to the interferer, causing the target component to be perceptually segregated from the remainder of the complex, thereby impairing across-frequency comparisons. Experiment 1 of the present study showed that an interferer presented contralaterally for 200 ms before and 100 ms after the signal complex (no simultaneous presentation) also impaired performance, but to a lesser extent than an ipsilaterally presented one. Experiment 2 showed that an interferer which was presented dichotically with an interaural level difference (ILD) of 10 dB, so that it was perceived contralaterally, had the same (large) effect as if it were presented ipsilaterally. Experiment 3 showed that, in the absence of any interferer, performance was impaired when the nontarget components were presented contralaterally to the target component. However, performance was not impaired when the nontarget components were presented dichotically with an ILD of 20 dB, so that they were perceived contralaterally to the target component. It is concluded that the level of performance in the mistuning task is determined by whether the target is presented to the same ear as the rest of the complex, rather than by its perceived location. © 1998 Acoustical Society of America.