Monaural masking patterns were obtained for pure tones spaced by octaves from 250 cps to 8000 cps at 20–100 db SL on three listeners in an attempt to provide extensive data assessing the relative importance of aural harmonics and cochlear spread of masking tone activity in the extension of masking to frequencies above the masking tone. The masking patterns confirm and extend the results of others, but especially indicate that second peaks do not necessarily occur at the second harmonic of the masking tone. The masking patterns are explained in terms of (1) the activity pattern of the masking tone in the cochlea, (2) beats between signal and masking tones, (3) aural harmonics, and (4) suppression of cochlear response to the signal.