Frequency selectivity was examined in three monkeys (Macaca nemestrina) by means of psychophysical tuning curves which were obtained under both simultaneous- and forward-masking conditions. The results showed that tuning curves obtained under the two masking conditions were qualitatively similar, sharing many of the same basic properties--sharply tuned tip segments, and greater selectivity at high frequencies than at low frequencies. As measured by Q10dB values, however, the forward-masking tuning curves were more sharply tuned than the simultaneous-masking curves. Additionally, the two procedures resulted in different conclusions regarding how frequency selectivity varies as a function of probe frequency. The simultaneous-masking results suggested that frequency selectivity improves up to at least 16 kHz, the highest frequency tested. The results from forward masking showed frequency selectivity to be greatest in the frequency region of 1-4 kHz. The results are discussed in terms of possible differences between the procedures, and in terms of their relationship to previous studies of psychophysical tuning cures in other species.