In order to determine the influence of shear velocity (U*), bed roughness (k) and moving particle diameter (d) on particle bed-load transport, velocities of single spherical glass beads (Vp) moving over beds of rhombohedrally packed spherical glass beads were determined under controlled laboratory conditions for several values of U*, k and d. A relationship between the position of zero water velocity (z1) and bed roughness (k) is established. Three “stages” of particle motion are recognized from a plot of Vp on d. The degree of development of each is a function of all three major variables. An empirical solution is offered where Vp is considered a function of (U*, k and d. A more detailed consideration of the forces which tend to propel and retard a moving particle leads to the development of a semi-empirical expression for the velocity of a single particle in contact-load motion. —A list of symbols is found at the end of the paper.