How quickly can a listener focus on a single tonal cue that indicates the frequency of an upcoming signal? Initial measurements were made with frequency uncertainty (signal frequency varies randomly from trial to trial) and with certainty (same frequency on all trials). Measured by a yes-no procedure, thresholds for 40- and 20-ms signals presented in continuous broadband noise at 50 dB SPL were higher in uncertainty than in certainty; the difference decreased monotonically from 5 dB at frequencies below 500 Hz to under 3 dB above about 2500 Hz. This decrease in the detrimental effect from uncertainty, which comes about with increasing signal frequency, may result from preferential attention to higher frequencies. In a second experiment, frequency again varied randomly, but each trial now began with a cue at the signal frequency. The critical variable was the delay from cue onset to signal onset. A delay of 352 ms eliminated the detrimental effect of frequency uncertainty at all frequencies. At the shortest delays of 52 and 82 ms the detrimental effect was reduced primarily at lower frequencies. Our analysis suggests that shifting focus to a cued frequency region, under optimal stimulus conditions, requires less than 52 ms.