Energetic requirements of under-water swimming in pygoscelid penguins were studied in Antarctica, using respirometry together with a 21 m long swim canal and externally attached devices recording the swimming speed and dive duration of unrestrained animals. Field measurements were compared with measurements of the hydrodynamic properties of an Adelie penguin model in a circulating water tank. Minimium transport costs during underwater swimming in Adelie (Pygoscelis adeliae), chinstrap (P. antarctica) and gentoo (P. papua) penguins averaged 4.9, 3.7 and 7.6 J kg-1 m-1, respectively, at their preferred swimming speeds of 2.2, 2.4 and 1.8 m s-1, allowing the birds to dive aerobically for 110, 130 and 93 s, respectively. From the swim canal measurements, we calculated a drag coefficient (CD) of 0.0368 for a typical Adelie penguin at 2.2 m s-1. This value is significantly lower than the CD of 0.04 of an ideal spindle and the CD of 0.0496 measured on the model in the laboratory. The reasons for this difference are discussed.