The at-sea summertime distribution of marine mammals between South Africa and Antarctica was determined along eight transects surveyed between December 2007 and January 2012. During 1930 30-minute transect counts, 1390 marine mammal individuals were attributed to 19 species: eight toothed whales (Odontoceti), six pinnipeds, and five baleen whales (Mysticeti). An additional two toothed-whale species were encountered ‘out of effort’. The four most numerous species accounted for 85% of the total number of individuals encountered: crabeater seal (Lobodon carcinophagus), humpback whale (Megaptera novaeangliae), Antarctic Minke whale (Balaenoptera bonaerensis) and fin whale (B. physalus). The distribution of these species was related to oceanographic features, such as water masses and fronts, pack ice and ice edge: These differences were statistically highly significant. Biodiversity was compared with other polar marine ecosystems.