A positional servo-mechanism was designed to facilitate multi-layer electronic control of complex orthotic and prosthetic devices. The design aim was to obtain adequate performance of the system using lightweight, flexible and inexpensive components. The mechanism is actuated by an antagonistic pair of McKibben pneumatic muscles. The muscles are controlled by a pair of twin electro-pneumatic valves operating in an on/off regime. System elements are described in the paper and some relevant design factors pointed out.