IN the course of some psycho-acoustic experiments it was found that one observer's thresholds of acuity at two frequencies separated by only a few tens of cycles differed by nearly 10 decibels. Dips in an audiogram are not uncommon; but the difference observed pointed to an uncommonly sharp change of sensitivity. Further tests revealed other equally sharp fluctuations in this observer's audiogram. Eventually a systematic exploration of the right-ear thresholds of this and one other observer was undertaken. Thresholds were measured at 10-cycle intervals over the range 400–3,000 c./s.