Animal studies in basal cochlear regions have shown that basilar-membrane phase curvature (or rate of change of group delay with frequency) is negative around characteristic frequency (CF), but near zero well below CF. This study examined whether psychophysical masking experiments in humans show the same difference between on- and off-CF phase curvature. Masked thresholds were measured for a 2-kHz signal in the presence of harmonic tone complex maskers with a fundamental frequency of 100 Hz, band-limited between 200 and 1400 Hz (off-frequency masker) or between 1400 and 2600 Hz (on-frequency masker). The results from four normal-hearing listeners are consistent with predictions from animal physiological data: negative phase curvature is found for the on-frequency masker, whereas the phase curvature for the off-frequency masker is near zero. The method and results provide a strong test for the temporal response of computational models of human cochlear filtering.