This paper presents experiments with a new, three-dimensional bipedal running behaviour for our robotic hexapod, RHex. The robot and the bipedal gait are underactuated, using only one actuated degree of freedom per compliant leg. We doubled up RHex's hind legs by attaching a duplicate set of hind legs at 180deg, forming 'S' shaped legs. This reduces the actuator speed requirements during non-contact, while preserving the bipedal dynamics and control challenges. Stable running at average speeds between 0.67 and 1.07 m/s with a success rate of 100% over thirty runs is obtained with only leg angle and body orientation feedback