To determine the role of hysteresis in propagating acoustic waves in soils, the stress/strain and sound speed/strain behaviors of soils are studied in triaxial cell tests. A series of quasi-static hysteresis loops with strain amplitudes ranging from 3 ¿ 10-5 to 2 ¿ 10-4 are formed during the tests. The slope of a hysteresis loop is compared with the dynamic elastic modulus from a sound speed measurement. It is found that the hysteresis slope linearly correlates with the dynamic modulus and decreases with the strain amplitude of the hysteresis loop. This discovery supports the hypothesis that acoustic wave propagation in a soil is a hysteresis process. Hysteresis might be the major source of the acoustic nonlinearity of soils.