A forced-choice observer-based testing procedure was used to determine pure-tone hearing thresholds for 2- to 4-week-old infants. Stimuli were 500-ms tone bursts of 500, 1,000, or 4,000 Hz with 500-ms intervals between tone bursts. Stimuli were presented monaurally by means of an insert earphone. Each 15-s trial consisted of 5 tone bursts, followed by a 5-s silent interval, followed by a 5-s test trial. During the test interval, either five tone bursts were presented, at the same intensity as in the first interval, or 5 s of silence were presented. An observer with no prior knowledge of trial type judged whether a sound had occurred during the test interval on the basis of the infant's behavior. Intensity varied between 25 and 70 dB SPL. For all infants at each frequency, psychometric functions were fit to the proportion of correct judgments as a function of intensity by probit analysis. Threshold calculated from this function was 56 dB at 500 Hz and 41 dB at 4,000 Hz. Threshold could not be calculated at 1,000 Hz; the observer made 0.65 correct judgments at the lowest intensity used. Results, which agree with other recent reports, suggest that the function relating sensitivity to sound frequency is not adultlike at this early age. (Author/RH)