ABSTRACT Young (mean mass 735 g) green turtles (Chelonia my das) were able to swim in a water channel at sustained speeds between 0·14 and 0·35 m.s−1. Oxygen consumption at rest was 0·07kg−1. h−1; at maximum swimming speed oxygen consumption was 3–4 times greater than at rest for a given individual. In comparison with other animals of the same body mass the cost of transport for the green turtle (0·186 1 O2.kg−1. km−1) is less than that for flying birds but greater than that for fish. From drag measurements it was calculated that the aerobic efficiency of swimming was between 1 and 10 %; the higher efficiencies were found at the higher swimming speeds. Based upon the drag calculations for young turtles, it is estimated that adult turtles making the round-trip breeding migration between Brazil and Ascension Island (4800 km) would require the equivalent of about 21 % of their body mass in fat stores to account for the energetic cost of swimming.