This study measured difference limens for fundamental frequency (DLF0s) for a target harmonic complex in the presence of a simultaneous spectrally overlapping harmonic masker. The resolvability of the target harmonics was manipulated by bandpass filtering the stimuli into a low (800-2400 Hz) or high (1600-3200 Hz) spectral region, using different nominal F0s for the targets (100, 200, and 400 Hz), and different masker F0s (0, +9, or -9 semitones) relative to the target. Three different modes of masker presentation, relative to the target, were tested: ipsilateral, contralateral, and dichotic, with a higher masker level in the contralateral ear. Ipsilateral and dichotic maskers generally caused marked elevations in DLF0s compared to both the unmasked and contralateral masker conditions. Analyses based on excitation patterns revealed that ipsilaterally masked F0 difference limens were small (<2%) only when the excitation patterns evoked by the target-plus-masker mixture contained several salient (>1 dB) peaks at or close to target harmonic frequencies, even though these peaks were rarely produced by the target alone. The findings are discussed in terms of place- or place-time mechanisms of pitch perception.