We describe a novel design for a compliant arm that can be mounted on a mobile robot. The main features of the arm are the in-parallel architecture of the arm and a novel control scheme that allows us to easily control the Cartesian stiness or impedance in the plane. Because the arm is compliant, a mobile robot can manipu- late or interact with objects that are not precisely positioned in the environment. Further, a mobile robot equipped with such an arm can cooperate with other mobile robots in manipula- tion tasks. For example, two such arms can hold an object in a stable grasp by applying and maintaining appropriate contact forces with the appropriate stiness. We present experimental results that show the performance of the compliant arm and the use of the arm while two platforms cooperate in a manipulation task.