The frequency of vortex shedding from a circular cylinder in oscillatory flows in a pipe is measured. The flow oscillations are induced by means of a rotatable butterfly valve set downstream of the test section. It has been observed that the frequency of vortex shedding varies with both the oscillation-amplitude parameter ΔU/Fd and the Reynolds number Re under constant oscillation frequency. In the case of a large amplitude parameter the frequency of vortex shedding synchronizes with the oscillation frequency of the flow and with one-half of it. The Strouhal number St is nearly constant for a small ratio of oscillation amplitude to the mean velocity ΔU/U, irrespective of the Reynolds number. For large ΔU/U, St decreases with increasing ΔU/U and /or decreasing Re.