A small, ducted counter-rotating coaxial rotor system was designed and tested for potential application to an unmanned, micro aerial vehicle (MAV). The objectives included a determination of the performance characteristics as a function of rotor-rotor spacing, rotor position in the duct, duct lip shape, and blade tip/duct wall clearances. Measurements were obtained as functions of the rotational speed hi hovering flight, axial climb (propeller mode), and in edgewise forward flight (helicopter mode) at various shaft angles of attack. The results showed that, in general, enclosing the coaxial rotor in the duct decreased the net rotor thrust for a given power and decreased its efficiency relative to the unducted coaxial. However, the aerodynamic forces on the duct itself recovered the net system thrust close to the isolated coaxial performance and also recovered efficiency. Measured figures of merit in hover were about 0.6, with propulsive efficiencies in axial flight of up about 0.75. The efficiency of the ducted coaxial was generally not as good as the unducted coaxial rotor system or the unducted/ducted single rotor. The results showed sensitivities to Reynolds number. The performance of the ducted coaxial system was insensitive to rotor-rotor spacing, but dependent on the position of the rotors within the duct Duct shape was found to have a notable effect on system performance, with the symmetric leading edge increasing both maximum static thrust and the aerodynamic efficiency. Increasing the tip gap in some cases was found to improve the performance of the ducted coaxial. Performance characteristics for all configurations were improved in low-speed edgewise forward flight. While the power loading of the ducted coaxial rotor system was generally lower than that of the ducted single rotor, the advantages of the former concept for a MAY design are still attractive in terms of thrust produced and aerodynamic efficiency for a given rotor size.