Experiments have been performed in a high-speed wind tunnel to determine threshold friction velocities for the onset of continuous particle motion (the fluid threshold velocity per Bagnold <1941>) for two types of gravel soil beds. The 50% finer-by-weight particle diameter (D¿50) for the fine gravel sample was ~6.35 mm and D¿50 for the coarse gravel bed was 42 mm. Measured threshold friction velocities (u*t), as determined with rake-mounted Pitot-static probes, were 3.1 and 9.9 m/s for the fine and coarse gravel soils, respectively. Corresponding free-stream threshold velocities (u∞t) were 41.8 and 94.3 m/s. Summarized results from the current measurements and other natural soil experiments <Gillette, 1978, 1988> vary approximately with the square root of particle diameter, in agreement with the prediction of Bagnold <1941> but exceed his results by a factor of 2--3. ¿ 1999 American Geophysical Union