The frequency of the resonance peak of the external auditory canal was measured in 250 children aged 3 to 12 years. The mean resonance peak frequency was greatest in the younger age groups (3089 Hz in 4 year olds, range 2298 to 3763) and then decreased with age until 7, the age at which adult values were reached. Children less than 7 years of age have mean resonance frequencies significantly different from adults, suggesting that the "adult" length of external auditory canal is not reached until the age of 7. This alteration in resonance frequency with age may have practical implications by affecting the insertion gain of hearing aid systems in children.