Three experiments were conducted to measure the decrement in performance that results from uncertainty in the time of onset of a signal presented against a continuous background of noise. The fixed‐interval observation experiment was employed. A light defined an observation interval for the listener during which the signal, a tone of 1000 cps, either was or was not presented [p(SN)=0.5]. The signal, when presented, started at an instant randomly selected within the observation interval. Thus, the listener was uncertain as to (1) whether or not the signal would occur in the observation interval, and (2) the onset time of the signal, if in fact the signal occurred. The interval of time uncertainty (ITU) during which the tone might start was systematically varied from one series of trials to the next, and the listener knew the duration of ITU in each series. After each observation interval, the listener indicated his confidence that a tone was presented by using a rating scale. Operating characteristics [p(y|SN) againstp(y|N)] were plotted on normal‐normal coordinates, and measures of detectability were computed. The functional relation between the detectability indexd, and the interval of time uncertainty is presented for each experiment.