Energy requirements of beavers (Castor canadensis) swimming voluntarily underwater were investigated in Neumünster Zoo (Germany) in a covered, still-water swim channel with oxygen and carbon dioxide respirometry. During the experiments, all activities of the beavers were monitored and recorded. While at rest within their thermoneutral zone on land (17 degrees C), beavers had a respiratory quotient of 0.95 and a resting metabolic rate of 1.58 W kg-1. When resting in water, energy requirements rose to 2.31 W kg-1. When swimming underwater in the channel, beavers preferred a mean speed of 0.64 m s-1, and their energy requirements rose to 2.64 W kg-1. Cost of transport, however, was minimal at 0.9 m s-1 and amounted to 0.36 J N-1 m-1. Although beavers must compromise form and function to operate on water and on land, their energy requirements while diving amount to only 1.65 times the resting metabolic rate and compare well with those of accomplished swimmers such as aquatic mammals and birds.