Off-frequency listening--the use of information in different frequency regions to improve performance in masking tasks--has been reported to influence psychoacoustical tuning curves measured in simultaneous masking. The present experiment was designed to establish whether suppression has an effect on off-frequency listening. Psychoacoustical tuning curves were obtained from three observers in both simultaneous and forward masking. A probe level of 10 dB SL and narrowband noise maskers were used. After obtaining the conventional tuning curves, a fixed masker, at either 1.8 or 2.2 kHz and 10 dB below the corresponding tuning-curve threshold, was added to limit off-frequency listening. In both simultaneous and forward masking, the branch of the tuning curve opposite the fixed masker was shifted downwards, but to a greater extent in forward masking. The suppression of the probe in tuning curves measured in simultaneous masking appears to constrain off-frequency listening. In forward-masking tuning curves, off-frequency listening has a greater effect, and may explain to a large extent the very sharp tuning which has been found in some studies.