The power output and propulsive efficiency of swimming bottlenose dolphins (Tursiops truncatus) were determined from a hydromechanical model. The propulsive movements were filmed as dolphins swam in large pools. Dolphins swam at velocities of 1.2-6.0 m s-1. Propulsion was provided by dorsoventral oscillations of the posterior body and flukes. The maximum angle of attack of the flukes showed a linear decrease with velocity, whereas the frequency of the propulsive cycle increased linearly with increasing velocity. Amplitude was 20 % of body length and remained constant with velocity. Propulsive efficiency was 0.81. The thrust power computed was within physiological limits. After correction for effects due to swimming depth, the coefficient of drag was found to be 3.2 times higher than the theoretical minimum assuming turbulent boundary conditions. The motions of the body and flukes are primarily responsible for the increased drag. This analysis supports other studies that indicate that bottlenose dolphins, although well adapted for efficient high-performance swimming, show no unusual hydrodynamic performance.