Discrimination thresholds for the interaural-phase difference (IPD) of a 500-Hz sinusoid were measured in the presence of a diotic notched-noise masker as a function of notch width. The reference sinusoid had zero IPD. The level was set to 3 dB above the individual masked threshold for the diotic sinusoid at the respective notch width. Just-noticeable IPDs increased from about 20° for notch widths up to 100 Hz to 100° at the largest notch width of 800 Hz. Model simulations show that the data are consistent with the equalization-cancellation theory, if effectively wider binaural filters are assumed.