The basic idea of a new type of actuator based on a shape memory alloy (SMA) and its application to underwater mobile robots are presented. The actuator consists of a multimode SMA, so that any shape can be produced as the synthesized sum of excited modes. Thus, the actuator shows the characteristics of distributed-parameter-type actuators with a specified number of modes. A flat-belt-type SMA is employed, and its bending characteristics are examined. A method of waveform synthesis is presented. Applying the proposed distributed actuators to the underwater mobile robot, a fish-fin-like mechanism is shown to obtain thrust forces in the water. The shape of the actuator was shown experimentally to be similar to the computed shape. As a result, the distributed actuator shows the possibility of application to distributed-parameter shape control.>