Optimum design procedures are devised to select the design parameters of a new class of buoyancy and gravity-driven underwater robots. In this class of robots, the natural forces of buoyancy and gravity are used to generate the torques necessary to move the arm. The resulting motions depend on the spatial shift of the arm's center of gravity with respect to its center of buoyancy. Maximizing the envelope of these motions while minimizing the arm total weight, by the proper choice of the design parameters, is essential to the efficient operation of the robot in an underwater environment. The procedures presented provide rational tools for designing this new class of underwater-compatible robots. (C) 1996 John Wiley & Sons, Inc.