Frequency difference thresholds were determined for fourteen 4- to 9-month-old infants (mean age, 6 months 10 days) using a discrimination learning paradigm, following a one-up, two-down staircase procedure. The subject heard 500 msec tone bursts repeated at a rate of one per sec, with a fixed standard frequency. At various points in this pulse train, the frequency of the tone burst changed for 6 sec. If the infant turned his head 45 degrees to his right within the 4 sec response interval following the change in frequency, a visual reinforcer was activated for 2 sec. Once the infant had learned the task with a relatively large frequency change, (96 Hz), the size of the frequency change was systematically decreased on each subsequent trial until the infant failed to turn, then increased again, and so on, in order to obtain an estimate of the smallest difference in frequency that the infant could detect. Following two correct responses, the frequency difference was decreased by one step; after one incorrect response the frequency difference was increased by one step. The results showed that infants could detect 2-3% changes in frequency at 1000, 2000, and 3000 Hz, while adults detected changes on the order of 1%. This finding supports the contention that infants could be using frequency difference as a cue in speech discrimination tasks. (Author/SS)