Despite the importance of fine-scale spatial use in understanding an animal's foraging ecology, these data cannot readily be collected for free-ranging marine mammals. We used an acoustic positioning system to quantify, for the first time, the fine-scale 3-dimensional (3D) spatial use of free-ranging Weddell seals swimming under ice. Unlike many other phocid species, lactating Weddell seals spent up to 25% of their time diving. Given the limited foraging range imposed on the seals by the fast ice upon which they breed, this could lead to prey depletion and even inter-specific competition. The seals focused their underwater activity on a relatively small region associated with a steep bottom contour, using the entire water column, with very little time spent at the bottom. This behaviour is consistent with feeding on bentho-pelagic prey such as Pleurogramma antarcticum. The 3D profile of individual dives consisted of the seals making simple, directed dives which gradually converged with the ocean floor. There was some variation from this pattern, usually associated with increased searching time. Such focused foraging activity may result in local prey depletion and intra-specific competition.