Two hydromechanical models are employed to analyse the motions of the caudal fin of a small specimen of Ostracion lentiginosum (Ostraciidae). Values of the drag coefficient of the body of the fish are inferred by equating the impulse of the thrust force produced over the tail beat cycle with the impulse of the drag force acting on the body over the same period of time. On average, the inferred values for the drag coefficient are within 15% of experimentally determined values. Drag coefficients for ostraciiform fish are an order of magnitude greater than those for streamlined fish. The hydromechanical efficiency of the caudal fin propeller of O. lentiginosum is calculated to be of the order of 0.5. This result is predicted by the hydromechanical theory.