This paper describes a single degree-of-freedom active-knee transfemoral prosthesis to be used as a test bed for the development of architectures for myoelectric control. The development of an active-knee transfemoral prosthesis is motivated by the inability of passive commercial prostheses to provide the joint power required at the knee for many activities of daily living such as reciprocal stair ascent, which requires knee power outputs of up to 4 W/kg. Study of myoelectric control based on surface electromyogram (EMG) measurements of muscles in the residual limb is motivated by the desire to restore direct volitional control of the knee using a minimally-invasive neuromuscular control interface. The presented work describes the design of a transfemoral prosthesis prototype including the structure, actuation, instrumentation, electronics, and real-time control architecture. The performance characteristics of the prototype are discussed in the context of the requisite knee energetics for a variety of common locomotive functions. This paper additionally describes the development of a single-subject diagnostic socket with wall-embedded surface EMG electrodes and the implementation of a control architecture for myoelectric modulation of knee impedance. Experimental results of level walking for a single subject with unilateral transfemoral amputation demonstrate the potential for direct EMG-based control of locomotive function.