Momentum flux considerations are used to formulate a differential equation governing the growth, with distance, of the mean longshore current velocity in the surf-zone on a plane, impermeable beach due to monochromatic waves. The equation is solved for the flow situation downstream of a surf-zone barrier. Experiments are performed which verify this solution at least at laboratory scale. The asymptotic (uniform flow) form of the relation is also shown to be in good agreement with the field and laboratory data of other investigators. Conclusions are reached governing the size of laboratory models necessary to represent conditions of fully developed longshore currents. (Author).