The swimming actions and performances of young green turtles (Chelonia mydas) and freshwater chelonians of similar size (Mauremys caspica, Chrysemys scripta elegans and Kinosternon subrubrum) were compared to evaluate the advantages conferred by the hypertrophied flapping forelimbs and streamlined form of the marine species. Evidence is presented to demonstrate that propulsive force is generated both on the upstroke and the downstroke of the green turtle's foreflippers, and that the force is produced largely by hydrodynamic lift generated as the flippers move through the water at relatively modest angles of attack. The green turtle's forelimbs therefore act like wings rather than as oars or paddles. Specimens of Chelonia mydas can generate twice the propulsive force, and swam six times as fast as Mauremys caspica. Much of this advantage stems from the efficiency of the swimming action (much less negative thrust is generated by C. mydas than by M. caspica) together with the streamlined shape, which also permits effective gliding. The swimming performance of young green turtles is similar in efficiency to that of streamlined fish such as mackerel.