A dynamic model for a swimming scallop was developed which integrates the mechanical properties of the hinge ligaments, valve inertia, the external fluid-flow reaction, the fluid pressure in the mantle cavity and the muscle contraction. Kinematic data were recorded for a swimming Placopecten magellanicus from high-speed film analysis. Dynamic loading experiments were performed to provide the required mechanical properties of the hinge for the same species. The swimming dynamics and energetics based on data from a 0.065 m long Placopecten magellanicus at 10 °C were analyzed. The main conclusions are as follows. 1. The mean period of a clapping cycle during swimming is about 0.28 s, which can be roughly divided into three equal intervals: closing, gliding and opening. The maximum angular velocity and acceleration of the valve movements are about 182 degrees s-1 and 1370 degrees s-2, respectively. 2. The hysteresis loop of the hinge was found to be close to an ellipse. This may be represented as a simple Voigt body consisting of a spring and dashpot in parallel, with a rotational stiffness of 0.0497 N m and viscosity coefficient of 0.00109 kg m2 s-1 for the 0.065 m long Placopecten magellanicus. 3. The external fluid reaction has three components, of which the added mass is about 10 times higher than the mass of a single valve, and the flow-induced pseudo-viscosity compensates for nearly half of the hinge viscosity for the 0.065 m long Placopecten magellanicus. 4. The locomotor system powered by the muscle can be divided into two subsystems: a pressure pump for jet production and a shell-hinge/outer-fluid oscillator which drives the pumping cycle. The dynamics of the oscillator is determined predominantly by the interaction of the external fluid reaction and the hinge properties, and its resonant frequency was found to be close to the swimming frequencies. 5. The momentum and energy required to run the oscillator are negligibly small (about 1 % for the 0.065 m long Placopecten magellanicus) compared with that for the jet. Almost all the mechanical energy from muscle contraction is used to perform hydrodynamic work for jet production. Thus, the Froude efficiency of propulsion in scallops is nearly the same as the entire mechanical efficiency of the locomotor system. This could be a fundamental advantage of jet propulsion, at least for a scallop. 6. The estimated maximum muscle stress is about 1.06x10(5) N m-2, the cyclic work is 0.065 J and power output is 1.3 W. Using an estimate of the mass of an adductor muscle, the work done by the muscle per unit mass is 9.0 J kg-1 and the peak power per unit mass is 185 W kg-1. 7. The time course of the force generation of the contracting adductor muscle is basically the same as that of the hydrodynamic propulsive force.