Abstract: An extendible wrist mechanism comprises a base and a spaced end plate to which an end effector or other tool holder may be mounted. Three linear actuators are disposed about the central axis directed from the base to the end plate. Each actuator is joined to the base by a joint permitting pivoting toward and away from the central axis and pivoting tangentially of a circle centered about the central axis. Each linear actuator is mounted to the end plate by a joint permitting pitch, yaw, and roll of the end member with respect to the linear actuator. A tube extends from the base and telescopingly receives a member which is attached to the end plate at the central axis by a U-joint permitting pitch and yaw of the end plate. The base mounted tube is keyed to the end plate mounted member so that the end plate is held against rotation with respect to the base.