A method to reduce the rolling motion of a ship in a seaway by means of actively controlled fins is described. The method is based on a multidisciplinary simulation: 1) the motion of the ship in a seaway is simulated by a numerical model called LAMP (Large-Amplitude-Motion Program), which is based on a source-panel method to model the flowfield around the ship, 2) the forces on the fins are simulated by a general unsteady vortex-lattice method, and 3) the commands to the fins are generated by a neural network. The ship is considered to be a rigid body and the complete equations of motion are integrated numerically in the time domain so that the motion of the ship and the complete flowfield are calculated simultaneously and interactively. The neural network controlling the fins can be progressively trained by a new moment-matching strategy. The results show substantial reductions in roll.