Scallops swim by rythmic opening and closing of the valves. Water ejected from the mantle cavity during closing of the valves generates thrust that propels the animal. The closing phase is powered by contraction of adduction muscles, whereas refilling is powered by energy stored in elastic material (abductin) compressed during the closing phase. This paper examines the physics of these oscillations. A model is developed to represent the oscillations with a second-order linear differential equation with constant coefficients. Values for the constants are measured experimentally or estimated from simple engineering principles. The system is forced by a function that represents the contraction of the adductor muscle, in this case a Fourier series defining a half wave rectified sine wave. The response of the system shows that there is a resonance at a frequency that is within the range of frequencies at which the animals swim. This implies that scallops may swim at the resonant frequency of the shell–hinge system. Attempts are made to estimate the energetic advantages of the phenomenon.