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In the previous paper, a new theory was developed for computing the radiation forces on a catamaran by extending Newman's unified slender-ship theory. The present paper is a sequel and concerned with the prediction of wave-exciting forces and motions of a catamaran in waves. The forward-speed version of Haskind-Newman's relation is applied to compute the wave-exciting forces by use of only the radiation-problem solutions. The obtained calculation formula is related to the Kochin function to be computed from the outer solution, which is expressed with the source and doublet distributions along the centerline of each demi-hull. At zero speed, using Haskind-Newman's relation is theoretically exact and in fact the results are in excellent agreement with independent results by a more rigorous 3-D integral-equation method. In the forward-speed case, measurements of the heave and pitch exciting forces are conducted at Fn=0.15 and 0.3, using the same model as in the forced oscillation tests shown in the previous paper. The agreement between the measured and computed results is not that good particularly in the pitch exciting moment, but the computed hydrodynamic forces are used in studying the motion characteristics in waves of a catamaran. Although the hydrodynamic interactions between twin hulls affect greatly on the radiation and wave-exciting forces, the resulting motions are similar to those of a single-hull ship, suggesting a cancellation of twin-hull interaction effects between the right- and left-hand sides of ship-motion equations.