Simultaneous swimming kinematics and hydrodynamics are presented for American eels, Anguilla rostrata, swimming at speeds from 0.5 to 2 L s(-1). Body outlines and particle image velocimetry (PIV) data were collected using two synchronized high-speed cameras, and an empirical relationship between swimming motions and fluid flow is described. Lateral impulse in the wake is estimated assuming that the flow field represents a slice through small core vortex rings and is shown to be significantly larger than forces estimated from the kinematics via elongated body theory (EBT) and via quasi-steady resistive drag forces. These simple kinematic models predict only 50% of the measured wake impulse, indicating that unsteady effects are important in undulatory force production. EBT does, however, correctly predict both the magnitude and time course of the power shed into the wake. Other wake flow structures are also examined relative to the swimming motions. At all speeds, the wake contains almost entirely lateral jets of fluid, separated by an unstable shear layer that rapidly breaks down into two vortices. The jet's mean velocity grows with swimming speed, but jet diameter varies only weakly with swimming speed. Instead, it follows the body wavelength, which changes more among individuals than at different speeds. Circulation of the stop-start vortex, shed each time the tail changes direction, can also be predicted at low speeds by the integral of squared tail velocity over half of a tail beat. At high speeds, these kinematics predict more circulation than is actually present in the stop-start vortex. Finally, the cost of producing the wake, one component of the total cost of transport, increases with swimming speed to the 1.48 power, lower than would be expected if the power coefficient remained constant over the speed range examined.