This study investigated spectral differences for auditory brain stem response (ABR) clicks measured in infant and adult ear canals. Click stimuli were presented via insert earphones into the ear canals of 15 adults and 11 infants at 30 and 60 dB nHL. An ER-7C probe microphone fed the stimulus to a spectrum analyzer for analysis of spectral content. Infants had spectral peaks at higher frequencies (mean = 2339.77 Hz) than adults (mean = 1618.75 Hz). The infants also exhibited greater variability in their spectral peaks (SD = 669.37) than did adults (SD = 319.0). The total energy of the clicks was the same in infants’ and adults’ ears. These results indicate that click stimuli such as those used for ABR testing resonate in infant and adult ears differently, and that the resonance in infants is much more variable than the resonance of adults. These findings provide additional evidence for the need to use infant norms when screening infant hearing using the ABR. Similarly, the findings suggest that infant ABRs may be more correlated with higher frequency behavioral thresholds than those assumed for adults (Ear Hear 12 3: 180-183).