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A high sensitive ultrasonic method for measuring the surface velocity in a liquid is described. This method is based on the detection of the phase of a high frequency continuous ultrasonic wave (probe beam) reflected from the moving surface. The analysis shows that the main phenomenon is the interaction, through the acoustic nonlinearity parameter B/A of the fluid, between the reflected carrier wave and the low frequency pressure wave transmitted by the moving surface in the liquid. This interaction produces a phase-shift of the carrier proportional to the surface velocity and to the time delay undergone by the probe beam. Results of experiments carried out in water with a 30-MHz focused transducer probe are in good agreement with the analysis. Surface velocity smaller than 0.04 mm/s (i.e., mechanical displacements down to 3 pin) can be detected in a 5 MHz bandwidth. Lateral resolution of 0.5 mm has been achieved. Compared to optical techniques, this method has the advantages of compactness and of a low sensitivity to surface roughness.