Resting metabolic rate (RMR), energy requirements and body core temperature were measured during underwater swimming in great cormorants (Phalacrocorax carbo sinensis) at the zoological garden in Neumunster, Germany, using gas respirometry and stomach temperature loggers. We used a 13 m long still water canal equipped with a respiration chamber at each end. Birds swam voluntarily in the canal at a mean speed of 1.51 ms-1. Power input during underwater swimming averaged 31.4 W kg-1. Minimal costs of transport of 19.1 J kg-1 m-1 were observed at a speed of 1.92 m s-1. Body core temperature was stable in all birds within the first 60 min spent in the canal. After that, body temperature dropped at a rate of 0.14°C min-1 until the birds voluntarily left the water. Our data indicate that great cormorants spend 2.7 times more energy than Adelie penguins (Pygoscelis adeliae) during underwater swimming. This can be essentially attributed to their poor insulation, their mode of locomotion underwater and differences in streamlining. RMR on land was related to body mass via VO2=0.691 M0.755 (where VO2 is O2-consumption in litre h-1 and M is body mass in kg). In order to quantify the effects of external devices on energy consumption during underwater swimming, we tested a dummy data logger attached to the back of the cormorants as well as a ring on the leg. The ring had no apparent influence on the swimming energetics of the cormorants. In birds equipped with dummy loggers, swimming speed was not significantly influenced, but both power input and costs of transport increased by a mean of 19% for swimming speeds between 1.4 and 1.8 m s-1.