The effect of the ground on a rotor wake in forward flight has been investigated experimentally in the working section of an 8ft x Oft straight-through wind tunnel. A three bladed fully articulated rotor with a solidity ratio of 0.07 and diameter of 1.06m, powered by a hydraulic motor, has been tested at a height of 0.47 rotor diameter above a solid ground board which has an elliptical leading edge. Tests have been run at various low advance ratios (<0.1) with two collective pitch settings. A three-element hot wire anemometer probe has been used to measure the average value of the three components of velocity simultaneously in the forward half (advancing side) of the rotor wake and in the main stream surrounding it. The rotor wake and the ground vortices have been visualized by smoke. Surface flow patterns on the ground board have located the interaction region between the rotor wake and the oncoming flow on the ground board. Theoretical estimates of the flowfield based on Heyson's vortex cylinder model (Ref. 2) are compared with the experimental results. Both experimental results and theoretical estimates show that the ground-induced interference is an upwash and a decrease in forward velocity. The upwash interference' opposes the vertical flow through the rotor, and have large effects on the rotor performance in producing thrust. The streamwise interference decelerates the mainstream and becomes more noticeable as the wake boundary is approached.