Nonlinear seismic response of soil is studied by comparison of the frequency-dependent soil amplification functions calculated on weak and strong motion. Amplifications are obtained by dividing Fourier amplitude spectra of acceleration at the ground surface by the spectra at the depths of 11 and 47 m in a borehole. Observed weak and strong motion spectral ratios are compared with those theoretically deduced from the one of the nonlinear soil models adopted in geotechnical engineering. Significant deamplification of the strong motion having PGA (peak ground acceleration) greater than 0.16 g, relative to the weak motion, is exhibited by the experimental ratios in the frequency range that is consistent with the model prediction. Existence of the different frequency bands, in which specific features of the nonlinear soil response are recognized in the theory, is also confirmed. These results give evidence of that nonlinear response can be observed at soft sedimentary sites from the real strong motion data. ¿ American Geophysical Union 1994