Wind-tunnel experiments were conducted to investigate tail-rotor performance for flight conditions in ground effect where helicopter directional control deficiencies have been experienced. Tail-rotor thrust was measured on a small-scale helicopter for various wind speeds and azimuths. Measurements were made with the fuselage and fin removed to emphasize the primary interference effects of a main rotor in ground effect. For a main-rotor height/diameter ratio h/D=0.265, the largest thrust reductions occur when (i) the tail rotor rotates in the direction where the top blade moves forward, (ii) wind speeds are above 15 knots, and (iii) wind azimuths are between 50° and 180°. Opposite rotation of the tail rotor eliminates most of the adverse interference effects of the main rotor. Flow visualization with neutrally buoyant helium bubbles revealed a small, well-defined ground vortex core at moderate and high wind velocities. Interpretation of the measured data showed that the ground vortex and trailing vortex systems of the main-rotor flow field produced tail-rotor thrust perturbations that are due to rotational and axial interference with the tail rotor.