During anguilliform swimming at the surface, four half waves are present along the body of the yellow-bellied sea snake Pelamis platurus (Hydrophiidae). As in other anguilliform swimmers, wave amplitude increases towards the tail; however, the relative caudal amplitude of P. platurus is less than that of the aquatic snakes Natrix and Nerodia and the eel Anguilla . Kinematic analyses of near-surface swimming at 15 and 32cms −1 were made from high-speed ciné films, and Lighthill's bulk momentum hydromechanical model was used to calculate swimming thrust power at these two velocities. The total thrust power generated by a 51 cm snake is 3–641×10 −4 Js −1 at 15cms −1 and 29.877×10 −4 Js −1 at 32cms −1 , with respective Froude efficiencies of 79% and 81%. Subsurface swimming velocities are low (2–4cms −1 ), and snakes usually assume a posture in which the tail is elevated and the posterior part of the body assumes a nearly vertical orientation. Undulatory movements by snakes swimming in this posture involve torsional and rolling motions of the body which, through changes in the camber of the keel and body, may contribute to thrust.