SemiQuad is a prototyped walking robot with a platform and two double-link legs. Thus, it is a ve-link mechanism. The front leg models identical motions of two quadruped's front legs, the back leg models identical motions of two quadruped's back legs. The legs have passive (uncontrolled) feet that extend in the frontal plane. Due to this the robot is stable in the frontal plane. This robot can be viewed as a "virtual" quadruped. Four DC motors drive the mechanism. Its control system comprises a computer, hardware servo-systems and power ampli ers. The locomotion of the prototype is planar curvet gait. In the double support our prototype is statically stable and over actuated. In the single support it is unstable and under actuated system. There is no ight phase. We describe here the scheme of the mechanism, the characteristics of the drives and the control strategy. The dynamic model of the planar walking is recalled for the double, single support phases and for the impact instant. An intuitive control strategy is detailed. The designed control strategy overcomes the di culties appeared due to unstable and under actuated motion in the single support. Due to the control algorithm the walking regime consists of the alternating di erent phases. The sequence of these phases is the following. A double support phase begins. A fast bend and unbend of the front leg allows a lift-o of the front leg. During the single support on the back leg the distance between the two leg tips increases. Then an impact occurs and a new double support phase begins. A fast bend and unbend of the back leg allows the lift-o of the back leg. During the single support on the front leg the distance between the two leg tips decreases to form a cyclic walking gait. The experiments give results that are close to those of the simulation.