The results of experiments on masking of tones by narrow bands of noise at 200, 800, and 3200 c.p.s. are presented. These are interpreted in terms of the critical band hypothesis. Assuming a simple resonant characteristic for the aural frequency selectivity the shape of the maskingvs.frequency function is computed and gives a reasonably good fit in the narrow regions studied. The critical band widths are determined at frequencies 200, 800, and 3200 c.p.s., and the corresponding values ofQof the equivalent tuned circuit are given. Data are presented for masking of tones slightly removed from the masking noise band, and are shown in fair agreement with the computed values. The selectivity curves are shown to be consistent with available data on differential sensitivity to frequency.