Low-frequency oscillations in the nearshore velocity field due to the instability of the shear of a longshore current are termed shear waves. Laboratory experiments have been performed to test theoretical predictions of the dispersion relation and growth rates for shear instabilities under controlled conditions and to provide empirical data for model calibration. The results, obtained with a high-resolution spectral analysis technique (Maximum Entropy Method), show that it is possible to generate instabilities of significant amplitude in a wave basin of limited length. The measurements demonstrate that these instabilities can be attributed to the shear instability of the longshore current. Quantitative comparison showing good correspondence with model predictions has been performed. ¿ 1997 American Geophysical Union