The threshold of grain motion under wave-induced fluid motion has been studied using wave periods of two to ten seconds in a large laboratory wave channel. The sand beds consisted of well-sorted material of quartz density, and the experiments covered a grain-size range of 0.018 to 0.145 centimeters. In all cases the gram diameter was such that the flow was in either the hydraulically smooth or the transitional flow regime. The results correlate well with the data from the oscillating-bed experiments of Bagnold (1946), suggesting that the oscillating-bed technique satisfactorily reproduces near-bottom motions at least for this set of conditions. The data from both these studies best fit the non-dimensional equation gamma s T 2 /rho D = 290(d 0 /D) (super 4/3) (rho gamma s D 3 /mu 2 ) (super -1/9) where gamma s = (rho s - rho )g, rho s and D are the grain density and median diameter respectively, mu and p are the fluid viscosity and density respectively, T is the wave period, d 0 is the near-bottom orbital diameter per wave cycle, and g is the acceleration of gravity. For material of quartz density in sea water, this relation becomes T = 0.17(d 02 /D) (super 1/3) (sec).