The Seattle Robotics Society has published a nice article on the design and construction of an autonomous robot fish by a team of students from Cal-Poly. Dongle has a sonar sensor (adapted from a commercial fish finder) for obstacle avoidance. Propulsion is provided by two servos mechanically connnected in series to produce fish-like tail movements. For a brain, Dongle has a 68hc11-based BOTBoard running SBASIC code. The article includes photos, schematics, and source code.