An initial wind-tunnel test was conducted to investigate the aerodynamic interactions between a propeller slipstream and a supercritical wing at transonic Mach numbers. The primary independent variables examined included Mach number, wing lift coefficient, and slipstream Mach number and swirl. The interference effects were found to be weak functions of free-stream Mach number, wing lift coefficient, and slipstream Mach number; swirl was found to have a significant effect. At a free-stream Mach number of 0.8 and a lift coefficient of 0.5, incremental drag results for 7° of swirl (upwash inboard) and a slipstream Mach number of 0.87 indicate a penalty equivalent to a 0.024 loss in propeller efficiency. However, at 11° the drag increment was favorable and was equivalent to a 0.032 increase in propeller efficiency. Wing pressure data indicated the effects of the slipstream were essentially restricted to that section washed by the slipstream. The addition of power to the slipstream tended to move the wing shock aft slightly. Swirl had significant effects on the chordwise pressure distributions of the inboard section of the wing within the slipstream but only slight effect on the outboard section. Neither surface nor wake pressures showed signs of significant flow separation induced by the slipstream. Although the results of this initial test have identified some questions that must be resolved, the test has proved very valuable in defining the magnitude of the interaction. The penalties associated with the slipstream were found to be considerably less than anticipated, thus enhancing the overall potential of turboprops for high-speed cruise applications.