Recent experiments have shown that frequency selectivity measured in tone‐on‐tone simultaneous masking improves with increasing delay of a brief signal relative to the onset of a longer duration gated masker. To determine whether a similar improvement occurs for a notched‐noise masker, threshold was measured for a 20‐ms signal presented at the beginning, the temporal center, or the end of the 400‐ms masker (simultaneous masking), or immediately following the masker (forward masking). The notch width was varied systematically and the notch was placed both symmetrically and asymmetrically about the 1‐kHz signal frequency. Growth‐of‐masking functions were determined for each temporal condition, for a noise masker without a spectral notch. These functions were used to express the thresholds from the notched‐noise experiment in terms of the level of a flat‐spectrum noise which would produce the same threshold. In simultaneous masking the auditory filter shapes derived from the transformed data did not change significantly with signal delay, suggesting that the selectivity of the auditory filter does not develop over time. In forward masking the auditory filter shapes were sharper than those for simultaneous masking, particularly on the high‐frequency side, which was attributed to suppression.