During sustained bouts of diving, predators such as fur seals may adjust the time they spend diving to maximize the time they can spend foraging and minimize the time spent at the surface between dives. To examine this, swimming speeds and the time allocated to different parts of the dive cycle were measured in 10 adult female Antarctic fur seals, Arctocephalus gazella, while they were foraging at sea. Mean swimming speeds during diving ranged from 1·32 to 1·99 m/s and 90% of diving swimming speeds within individuals were between 1 and 2·5 m/s. This represented a narrower range of speed than was exhibited by animals when swimming at the surface. Swimming speeds were fastest during the descent and ascent phases of dives with a reduction in speed near the bottom of the dive, when the seals were assumed to be feeding on krill. Time spent at the surface increased as a curvilinear function of dive duration and was influenced by diving swimming speed. This relationship was, on average, close to that predicted by a model (Houston & Carbone 1992, Behav. Ecol., 3, 255–265) which suggested that the metabolic rate was greatest during the foraging phase of dives (5·9 times predicted BMR) than during the ascent or descent (3·6 times predicted BMR). Deep diving (>30 m) and high swim speeds also incurred costs in terms of reduced foraging time and may explain why mean dive depths for individuals were within the range of 12–33 m. These data broadly support the predictions of models of diving behaviour based on balancing the supply of, and demand for, oxygen and the principle that fur seals are attempting to maximize the time they spend within the foraging area.