Six adult Weddell seals, ranging in weight from 360 to 430 kilograms were released on separate occasions at a man-made ice hole in McMurdo Sound, Antarctica. Their voluntary diving behavior during the days and nights of the month of September was recorded. The seals spent 58 percent of their time diving in the day and 43 percent at night. The nature of the dives was strikingly different. The longest and deepest dives were recorded during the day. For every 100 hours of observation there were five dives > 50 minutes duration. No dives in excess of 50 minutes were observed at night. The deepest dive recorded in the daytime was 600 meters and the deepest at night was 230 meters. Dives of durations > 20 minutes are believed to be efforts to find other breathing holes. Such dives tend to be three times as deep in the daytime as at night. Finally, it was noted that at this time of year three times more seals were observed sleeping on the ice at night than in the day. It is concluded that vision is an important perceptive modality for under-ice orientation.