The rate of oxygen consumption and energetic cost of locomotion of captive harbor porpoises Phocoena phocoena were studied in Hokkaido, Japan, using respirometry together with externally attached devices (a data logger) that recorded swim speed and dive depth. Harbor porpoises swam freely in a tank at a swim speed ranging 0.5–4.2 m/s. Swimming during most dives was relatively slow with mean swim speed being 0.76–0.91 m/s. Oxygen consumption rate of the swimming harbor porpoise increased with swim speed according to a cubed function. The minimum cost of transport during underwater swimming in the harbor porpoise was 2.39–2.43 J/kg per m at an average swim speed of 1.3–1.5 m/s. However, porpoises usually swam slower to conserve energy and dived aerobically. This explains why harbor porpoises can dive repeatedly and continuously without extended rest at the sea surface.