This paper presents a biometric personal-authentication method that exploits acoustic characteristics of human ears. It transmits a probe signal into the ear and receives its reflection, which contains personal identity information about the shape of the ear canal. Based on a study of effective and efficient acoustic feature representation and the use of audio equipment suitable for acquiring features with low within-individual variability, the proposed method achieves a promising equal error rate of 0.97% with only 12 feature components. A prototype system for Android smartphones is also presented.