AbstractIn 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*,kandd.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 ofVpond.The degree of development of each is a function of all three major variables.An empirical solution is offered whereVpis considered a function of (U*,kandd.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.