Velocity measurements in thin oscillatory boundary layers require an instrument with high natural frequency and sensitivity, and small bulk at the point of measurement. An instrument which possesses these qualities has been constructed for use in two-dimensional flows. It consists essentially of a thin fibre suspended at right angles to the stream. As the velocity of the stream varies so does the tension in the fibre. Thus, measurement of the tension allows the velocity to be determined. The results obtained with this device have been very satisfactory.