This paper presents a control system for a powered transtibial prosthesis that provides stair ascent and descent capability, as well as ability for user-controlled transitions between walking, standing, stair ascent, and stair descent. The control system was implemented on a powered prosthesis and evaluated on a single unilateral transtibial amputee subject. The ability of the prosthesis to provide appropriate functionality during stair ascent and descent was assessed by comparing gait kinematics and kinetics of the prosthesis to those of a passive dynamic elastic response prosthesis and those of a set of non-amputee subjects. Data from the assessment indicates that the powered prosthesis is able to provide some desirable stair ascent and stair descent characteristics, relative to the passive prosthesis.