During the austral summer of 1963–64 an investigation was started near “Mc-Murdo” on the diving habits of the Weddell Seal (Leptonychotes weddelli). The Weddell Seal was selected for this investigation because of some unique characteristics of its behaviour and because of the environment in which it occurs at McMurdo Sound, Antarctica. For eight to ten months of each year the sea ice provides a solid cover except for tidal cracks along the coast, pressure cracks between old and new ice, and scattered holes cut for oceanographic work. This gives the seals rather specific areas where they may surface in order to breathe. When they visit these breathing holes, instruments may be attached to them with a high probability that they will revisit the same hole and allow the investigator to remove the instrument. With this knowledge of the Weddell Seal' diving behaviour, it was my intention not only to gain some idea of the maximum diving epths and submergence times, but also to obtain information on such things as rates of descent and ascent, cruising depths and submarine orientation patterns. To determine these various parameters two types of instrument were needed, a depth-time device for recording dive profiles and a smaller, more easily attached unit that could be used to measure maximum depth of dives of a large number of seals.