The energy requirements of Brandt's cormorants (Phalacrocorax penicillatus) during surface swimming were measured in birds swimming under a metabolic chamber in a water flume. From the oxygen consumption recordings, we extrapolated the metabolic rate and cost of transport at water speeds ranging from 0 to 1.3 m s(-)(1). In still water, the birds' mean mass-specific rate of oxygen consumption ( V(O2)) while floating at the surface was 20.2 ml O(2 )min(-)(1 )kg(-)(1), 2.1 times the predicted resting metabolic rate. During steady-state voluntary swimming against a flow, their V(O2) increased with water speed, reaching 74 ml O(2 )min(-)(1 )kg(-)(1) at 1.3 m s(-)(1), which corresponded to an increase in metabolic rate from 11 to 25 W kg(-)(1). The cost of transport decreased with swimming velocity, approaching a minimum of 19 J kg(-)(1 )m(-)(1) for a swimming speed of 1.3 m s(-)(1). Surface swimming in the cormorant costs approximately 18 % less than sub-surface swimming. This confirms similar findings in tufted ducks (Aythya fuligula) and supports the hypothesis that increased energy requirements are necessary in these birds during diving to overcome buoyancy and heat loss during submergence.