Acoustic wave detection of various underivatized amino acids that were hydrodynamically introduced into and electrokinetically migrated along a capillary tube has been achieved. The detection principle is based on the measurement of the ultrasonic absorption and density change of the aqueous amino acid samples as they pass by the detection zone. Acoustic wave detection of underivatized leucine was obtained in the ultrasonic frequency range of 100 kHz to 20 MHz. This was accomplished by measuring the insertion loss from the vector ratio of the signal voltage to the source voltage obtained at the generating and receiving piezoelectric transducers. Linear concentration dependence of the insertion loss of underivatized leucine was established. The effects of capillary internal diameter, capillary geometry (square and circular), buffer concentration, buffer pH and acoustic wave frequency on the insertion loss were investigated. Subsequently, separations of underivatized amino acids (leucine/histidine and leucine/tryptophan) were performed under different buffer conditions and different capillary geometry.